18
CHAPTER
Organic Chemistry
Chapter Preview Sections 18.1 Hydrocarbons MiniLab 18.1 How Unsaturated is Your Oil? 18.2 Substituted Hydrocarbons MiniLab 18.2 A Synthetic Aroma 18.3 Plastics and Other Polymers MiniLab 18.3 When Polymers Meet Water ChemLab Identification of Textile Polymers
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Spiders—Organic Chemists?
S
piders produce a silk composed of a protein called fibroin. Spider silk is an organic compound that is composed of amino acids. This compound is the same as the one produced by silkworms. The silk is very strong, elastic, and lightweight. Silk is one of over 10 million different organic compounds that contain carbon. Organic compounds are found in many products you use every day.
Start-up Activities What I Already Know
Making Slime In addition to carbon and hydrogen, most organic substances contain other elements that give the substances unique properties. In this lab, you will work with an organic substance consisting of long carbon chains to which many ⫺OH groups are bonded. How will the properties of this substance change when these groups react to form bonds called crosslinks between the chains?
Review the following concepts before studying this chapter. Chapter 4: bonding in covalent compounds Chapter 5: naming of hydrocarbons Chapter 6: combustion of hydrocarbons Chapter 9: bonding in hydrocarbons
Safety Precautions
Reading Chemistry Do not allow solutions or product to contact eyes or exposed skin.
Materials • 4% sodium tetraborate • disposable plastic cup • (borax) solution • stirring rod • 4% polyvinyl alcohol solution Procedure 1. Pour 20 mL of 4% polyvinyl alcohol solution into a small disposable plastic cup. Note the viscosity of the solution as you stir it. 2. While stirring, add 6 mL of 4% sodium tetraborate solution to the polyvinyl alcohol solution. Continue to stir until there is no further change in the consistency of the product. 3. Use your gloved hand to scoop the material out of the cup. Knead the polymer into a ball.
Observe the chemical structures that appear throughout the chapter. Note which elements are present in each. Next, look at the key terms for each section. As you read, match up the key terms with the chemical structure examples.
Preview this chapter’s content and activities at chemistryca.com
Analysis What physical property of the product differs markedly from those of the reactants?
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SECTION
18.1 SECTION PREVIEW Objectives ✓ Write and interpret structural formulas of linear, branched, and cyclic alkanes, alkenes, and alkynes. ✓ Distinguish among isomers of a given hydrocarbon. ✓ Infer the relationship between fossil fuels and organic chemicals.
Review Vocabulary Voltage: electrical potential difference, expressed in units of volts.
New Vocabulary saturated hydrocarbon alkane isomer unsaturated hydrocarbon alkene alkyne aromatic hydrocarbon fractional distillation cracking reforming
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H
ave you ever seen a spectrum of colors on the top of an otherwisedrab puddle of water? What causes those bright colors to appear? They form as a result of pollution, namely small amounts of gasoline or oil that have leaked out of automobiles and formed a miniature oil spill on the puddle. Gasoline is composed mostly of hydrocarbons, the organic compounds with the names and structures you studied in Chapters 5 and 9. Hydrocarbons have properties that are different from those of water. They are insoluble in water, which is why they form a distinct layer on a puddle of water. Hydrocarbons are less dense than water; that’s why the layer floats on top of the puddle. What causes the colorful effect? The spill forms an extremely thin layer of hydrocarbon molecules on the water, which reflects sunlight. In this section, you will learn more about the structures and names of hydrocarbons, as well as the sources of these useful compounds.
Millions and Millions of Organic Compounds Carbon is unique among elements in that it can bond to other carbon atoms to form chains containing as many as several thousand atoms. Because a carbon atom can bond to as many as four other atoms at once, these chains can have branches and form closed-ring structures that make possible an almost endless variety of compounds. In addition, carbon can bond strongly to elements such as oxygen and nitrogen, and it can form double and triple bonds. Thus, carbon forms an enormous number of compounds with chains and rings of various sizes, each with a variety of bond types and atoms of other elements bonded to them. Fortunately, you don’t need to study each of these millions of compounds to understand organic chemistry because they can be classified into groups of compounds that have similar structures and properties. Organic Chemistry
Saturated Hydrocarbons Gasoline is a mixture of organic compounds that is derived from petroleum. Most of the compounds in gasoline are hydrocarbons. You will recall learning that hydrocarbons are organic compounds containing only hydrogen and carbon atoms. A hydrocarbon in which all the carbon atoms are connected to each other by single bonds is called a saturated hydrocarbon. Another name for a saturated hydrocarbon is an alkane. Although burning alkanes for fuel is their most common use, they are also used as solvents in paint removers, glues, and other products.
Alkanes Alkanes are the simplest hydrocarbons. The carbons in an alkane can be arranged in a chain or a ring, and both chains and rings can have branches of other carbon chains attached to them. Alkanes that have no branches are called straight-chain alkanes. Methane, CH4; ethane, C2H6; propane, C3H8; and butane, C4H10 are all common fuels. Their structural formulas show that each differs from the next by an increment of —CH2—. H H
C
H
H
H Methane
H
H
C
C
H
H
H
H
H
H
H
C
C
C
H
H
H
Ethane
H
H
Propane
H
H
H
H
C
C
C
C
H
H
H
H
H
Butane
Some alkanes have a branched structure. In these compounds, a chain of one or more carbons is attached to a carbon in the longest continuous chain, which is called the parent chain. If a chain containing a single carbon is branched off the second carbon of a propane parent chain, a branched alkane with the following structure results. H H H
H
H
C
C
C
H C
H
2-methylpropane
H H H The carbon atoms in alkanes can also link up to form closed rings. The most common rings contain five or six carbons. The structures of these compounds can be drawn showing all carbon and hydrogen atoms. H H H H C
H Cyclopentane
C H
C
H C C
H HH H
H
H H H H
C C
C C C
C
H H Cyclohexane H H
H H 18.1
Hydrocarbons
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Cyclopentane
Cyclohexane
Figure 18.1 Components of Gasoline and Octane Rating Gasolines are rated on a scale known as octane rating, which is based on the way they burn in an engine. The higher the octane rating, the greater the percentage of complex-structured hydrocarbons that are present in the mixture, the more uniformly the gasoline burns, and the less knocking there is in the automobile engine. Thus, a gasoline rated 92 octane will burn more smoothly than one rated 87 octane.
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Structural diagrams Table 18.1 The First Ten Alkanes can be simplified by using straight lines to represent the bonds Formula Name between atoms in the CH4 methane rings. In these ring diaC2H6 ethane grams, each corner repreC3H8 propane sents a carbon atom. C4H10 butane Because carbon usually C5H12 pentane forms four bonds, it is C6H14 hexane understood that enough C7H16 heptane hydrogen atoms are C8H18 octane bonded to each carbon to C9H20 nonane give it four bonds. Rings C10H22 decane containing five and six carbons are drawn as a pentagon and a hexagon, respectively. Structural diagrams of straight- and branched-chain hydrocarbons also can be written in a simplified way by leaving out some of the bonds. For example, the formula for propane can be written as CH3—CH2—CH3. In this type of shorthand structure, the bonds between C and H are understood. In an even more simplified type of shorthand, the condensed structural formula for propane can be written as CH3CH2CH3. Here, the bonds both between C and C and between C and H are understood. These condensed structural formulas will be used throughout this chapter. Figure 18.1 shows the names and structures of some saturated hydrocarbons found in typical gasoline mixtures. The properties of a gasoline mixture—such as how well it burns in an engine—are determined by the relative amounts of various components present in the mixture. Hydrocarbons that contain rings and many branches burn at a more uniform rate than do straight-chain alkanes, which tend to explode prematurely, causing engine knock. The octane number on a gasoline pump is an index that indicates the relative amounts of branched and ring structures in that blend of gasoline.
Organic Chemistry
CH Pentane H2CH2 3 C H CH3C 2 CH H CH Decane H CH2C 2 3 3 C H C H C H CH CH2C 2 2 2 2 2
C y clo
CH
3
CH
hexane
H H H H
H
H
H
H
2 CH C 2 H2CH CH 2 3
H H H H
Hexane
Naming Alkanes The names of the first ten straight-chain alkanes, shown in Table 18.1, are used as the basis for naming most organic compounds. To name a branched alkane, you must be able to answer three questions about its structure. 1. How many carbons are in the longest continuous chain of the molecule? 2. How many branches are on the longest chain and what is their size? 3. To which carbons in the longest chain are the branches attached? For convenience, the carbon atoms in organic compounds are given position numbers. In straight-chain hydrocarbons, the numbering can begin at either end. It makes no difference. In branched hydrocarbons, the numbering begins at the end closest to the branch. Examine the structure of this branched alkane. CH3 1
3
2
The terms saturated and unsaturated originated before chemists understood the structures of organic substances. They knew that some hydrocarbons would take up hydrogen in the presence of a catalyst. When the substance would react with no more hydrogen, it was said to be saturated. Hydrocarbons that would take up additional hydrogen were said to be unsaturated. Today, we know that unsaturated hydrocarbons contain double and triple bonds that will react with hydrogen to form single bonds.
4
CH3CHCH2 CH3
Four carbons are in the longest continuous chain, so butane is the parent chain and will be part of the compound’s name. There is only one branch, and it contains one carbon. Instead of calling this a methane branch, change the -ane in methane to -yl. Thus, this is a methyl branch. Because the methyl branch is attached to the second carbon of the butane chain, this compound has the name 2-methylbutane. Now, examine the structure of a different hydrocarbon. CH3 1
2 3
CH3CCH3 CH3
Propane will be part of this compound’s name because the longest continuous chain has three carbons. Two methyl branches are present, both on the second carbon. To indicate the presence of more than one branch of the same kind, use the same Greek prefixes presented in Chapter 5 for naming hydrates and molecules. The prefix to use when two of anything are present is di-. Thus, the name of this compound is 2,2-dimethylpropane. Note that the positions where the methyl groups are attached to the parent chain are written, separated by a comma. Follow the three-step process to name the following alkane. CH3 CH3— CH2— CH — CH — CH2— CH3 CH2 CH2 CH3
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Hydrocarbons
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1. Count the number of carbons in the longest continuous chain, which may not always be written in a straight line. What looks like a branch may be part of the parent chain. Because this compound has a chain of seven carbons, heptane will be part of the name. 2. Note the size and number of branches present. The compound shown has one branch that is one carbon long and one branch that is two carbons long. Ethyl is the name given to a branch that has two carbons, and you know that the name for a one-carbon branch is methyl. If a compound has two different branches, they should be named in alphabetical order. Thus, part of the name will be ethylmethylheptane. 3. Number the carbons in the chain starting at the end nearest a branch. In the compound shown, position numbers will start on the right side. CH3 3
4
1
2
CH3— CH2— CH — CH — CH2— CH3 5
CH2
6
CH2
7
CH3
The position numbers of the carbons that have branches are 3 and 4. Now, put the name together. The position numbers of the branches come first, followed by a hyphen and then the name of the branch. The complete name for this compound is 4-ethyl-3-methylheptane. Alkanes containing rings are named using the same rules, but the prefix cyclo- is placed before the name. Cyclohexane has six carbons connected in a ring, and cyclononane has a nine-carbon ring.
PRACTICE PROBLEMS
For more practice with solving problems, see Supplemental Practice Problems, Appendix B.
1. Write the structural formulas for the following branched alkanes. a) 2-methylbutane b) 1,3-dimethylcyclohexane (Hint: Begin numbering at any carbon in the ring, then attach the methyl groups.) c) 4-propyldecane d) 2,3,4-trimethylheptane 2. Name each of the following alkanes. a) H
H
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H
H
C
C
H
H
H
C
H
H
H
C
C
C
H
H
H
H
b)
H H H
H
H
C
C
C
H
H
H
H
H
C
C H
C
H
H
H
H
H
H
H
H H C H
H
H
C
C
C
C
C
C
C
H
H
H
H
H
H C H H
H
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
c) H
d) H
H
C
H
H
C
H
H
H
H
Isomers Butane and 2-methylpropane are two alkanes with different names and different structures. H
H
H
H
H
H
C
C
C
C
H
H
H
H
Butane
H H
H
1
C
H
H
C 2
C
H
H
Figure 18.2
H 3
C
H
H
2-methylpropane
Both of these compounds are familiar fuels that burn to give off heat in handheld lighters, Figure 18.2. Is there any relationship between these two compounds? If you count the number of carbon and hydrogen atoms in each, you will find that they both have four carbons and ten hydrogens, which gives them the same molecular formula, C4H10. Compounds that
Disposable Lighter Fuel Although butane and 2-methylpropane are gases at room temperature and atmospheric pressure, they can be liquefied under higher pressures in closed containers. Most disposable lighters contain one or both of these compounds, which are flammable enough to be ignited by a spark. 18.1
Hydrocarbons
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CH3
CH3
— —
Properties of Pentane Isomers Structure and properties are closely related, as you can see by examining the isomers of pentane. Although all three compounds have the formula C5H12 , differences in the amount of branching affect their properties. Note the differences in the shapes of the molecules.
have the same formula but different structures are called isomers. Butane and 2-methylpropane are known as structural isomers. Each has the molecular formula C4H10, but they have different structural formulas because the carbon chains have different shapes. Despite their identical molecular formulas, isomers have different properties. The boiling and melting points of 2-methylpropane and butane are different, as are their densities and solubilities in water. In addition, their chemical reactivity is different. Figure 18.3 shows some property differences in the isomers of pentane.
—
Figure 18.3
CH3—CH2—CH2—CH2—CH3
CH3—CH2—CH—CH3
CH3—C—CH3
Pentane bp 36°C
2-methylbutane bp 28°C
CH3 2,2-dimethylpropane bp 9.5°C
Although only two alkane isomers have four carbons, the number of possible isomers increases rapidly as carbon atoms are added to the parent chain. This is because longer chains provide more locations for branches to attach. A methyl branch on a six-carbon parent chain can be attached to either the second or the third carbon from the end of the chain. To make sure that these two compounds are really isomers, count the number of carbon and hydrogen atoms in the structures formed by placing the methyl group at those two positions, and write the molecular formulas. H H H
1
C
H
H
C 2
C
H H
H 3
C
H 4
C
H H H 2-methylhexane C7H16
H 5
C
H
H 6
C
H
H H
H
1
C
H
H 2
C
H
H
C 3
C
H
H
4
C
H H 3-methylhexane C7H16
H
H
5
C
6
H
H
C
H
There are three isomers of pentane, five of hexane, and more than 4 billion isomers of the alkane with the formula C30H62. 628
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Properties of Alkanes Properties are affected by the structure or arrangement of atoms present in a molecule. Another factor that affects properties of alkanes is chain length. In general, the more carbons present in a straight-chain alkane, the higher its melting and boiling points. At room temperature, straight-chain alkanes that have from one to four carbon atoms are gases, those with from five to 16 carbon atoms are liquids, and those with more than 16 carbon atoms are solids. A property shared by all alkanes is their relative unreactivity. You will recall from Chapter 9 that the carbon-carbon and carbon-hydrogen bonds found in alkanes are nonpolar. Because alkanes don’t have any polar bonds, they undergo only a small number of reactions and will dissolve only those organic compounds that are nonpolar or that have low polarity, such as oils and waxes. The nonpolar and low-reactive nature of alkanes makes them good organic solvents. Paints, paint removers, and cleaning solutions often contain hexane or cyclohexane as solvents.
Four ways to represent molecules are shown here. At the top are ball-and-stick models, with springs representing double and triple bonds. At the bottom are space-filling models, which show the actual shape of the molecule. 䊳
Figure 18.4 Single, Double, and Triple Bonds Two carbon atoms can share one, two, or three pairs of electrons. In saturated hydrocarbons, carbon atoms share only one pair, whereas in unsaturated hydrocarbons, the carbon atoms participating in double or triple bonds share two or three pairs. Because a carbon atom forms four bonds, unsaturated hydrocarbons contain fewer than the maximum number of hydrogen atoms.
CH3—CH3
CH2=CH2
CH≠CH
HH HCCH HH
HC CH H H
HC CH
Ethane
Ethene
Ethyne
Unsaturated Hydrocarbons
Figure 18.5
You learned in Chapter 9 that carbon atoms in organic compounds can be connected by single, double, or triple bonds. A hydrocarbon that has one or more double or triple bonds between carbons is called an unsaturated hydrocarbon. Molecules with single, double, and triple bonds are compared in Figure 18.4.
Alkenes in Gasoline Gasoline contains some alkenes as well as alkanes. Alkenes burn more uniformly, and their presence raises the octane rating of a gasoline.
Alkenes Gasoline contains several hydrocarbons with double bonds. A hydrocarbon in which one or more double bonds link carbon atoms together is called an alkene. The most common alkenes found in gasoline are pictured in Figure 18.5.
Alk a n e
s
CH = CH2 – CH3 1– hexene CH – CH2– CH2 – 2 Cyclohexene
CH3 Toluene
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Hydrocarbons
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How unsaturated is your oil?
1
Most animal fats are saturated hydrocarbons and are solids at room temperature, whereas most vegetable fats are unsaturated and are liquids at room temperature. Both types of fats are essential in our diets, but medical research has shown that eating high levels of saturated fats can contribute to health problems such as heart disease. Different amounts of unsaturation in fats can be compared by testing how quickly a red-brown iodine solution added to each fat is decolorized. Iodine adds to carbons that take part in multiple bonds, forming colorless organic halogen compounds when the double or triple bond is broken. Procedure 1. Place 20 mL of peanut oil in one small flask and 20 mL of canola oil in another flask. Label the flasks. 2. Add five drops of tincture of iodine to each oil and swirl to mix well. Note the color of each solution. 3. Heat both flasks on a hot plate set on low. 4. Note which oil returns to its original color first. This oil is more unsaturated than the other.
5. Now read the food label on each bottle of oil, and determine whether your test results agree with how much unsaturated fat the labels say are in each oil. Analysis 1. What happens to the redbrown iodine when it is added to a relatively unsaturated oil? 2. Examine food labels on bottles of the oils listed below, and predict which of each pair will decolorize faster. a) canola or corn oil b) coconut or sunflower oil
Alkenes are named using the root names of the alkanes, with the -ane ending changed to -ene. The simplest alkene is ethene, CH2=CH2, which contains two carbons linked in a chain. Ethene, a gas at room temperature, is the most important organic compound used in the chemical industry. Almost half of the ethene used is converted into plastics. It is also used to make automobile antifreeze, ethylene glycol. Figure 18.6 shows another use of ethene. Figure 18.6 Ethene and Ethylene: One and the Same Ethylene is the common name of ethene. This compound occurs naturally as a plant hormone that functions to speed up ripening of fruits and vegetables. Unripe fruits and vegetables can be treated with ethylene so that they all ripen at the same time, making harvesting more efficient.
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The alkene with three carbons in a chain is called propene, CH2=CHCH3. When four or more carbons are present in a chain, the double bond can be located at more than one possible position. When naming these alkenes, a number must be added at the beginning of the name to indicate where the double bond is located. You should use the following steps in naming alkenes with long carbon chains. 1. Count the number of carbons in the longest continuous chain that contains the double bond, and assign the appropriate alkene name. 2. Number the carbons consecutively in the longest chain, starting at the end of the chain that will result in the lowest possible number for the first carbon to which the double bond is attached. 3. Write the number corresponding to the first carbon in the double bond, followed by a hyphen and then the alkene name. What is the name of the compound that has the following structure? CH2=CHCH2CH3
cis: cis (L) on this side In a cis configuration, certain groups of atoms are located on the same side of the molecule. trans: trans (L) across In a trans configuration, certain groups of atoms are located on opposite sides of the molecule.
This compound has four carbons in a chain with one double bond, so butene will be part of its name. Numbering the carbons starting on the left side of the compound gives the first carbon that is part of the double bond the position number of one. Thus, this compound is 1-butene. 1
2
3
4
CH2=CHCH2CH3 1-butene An isomer of 1-butene is 2-butene, CH3CH=CHCH3. These two compounds have the same molecular formulas but different structures. They are called positional isomers because they differ only by the position of the double bond. Positional isomers have different properties, just as structural isomers do. The formation of a double bond prevents the carbons on each side of the bond from rotating with respect to each other. If the two groups attached to either carbon are different, the alkene can have two different geometric structures. These structures are geometric isomers. Study the two geometric isomers of 2-butene that are modeled in Figure 18.7. In the isomer called cis-2-butene, the hydrogen atoms and —CH3 groups are on the same side of the double bond. In trans-2-butene, the hydrogen atoms and —CH3 groups are on opposite sides of the double bond.
The properties of a pair of geometric isomers vary. Trans isomers are more symmetrical than cis isomers, and they can pack together more closely when in the solid state. This close packing makes the molecules harder to pull apart. As a result, trans isomers generally have higher melting points than do cis isomers. 䊲
Figure 18.7
—
CH3
C=C
—
—
—
H
H —
—
C=C
H
CH3
—
CH3
—
Geometric Isomers Cis-2-butene and trans-2butene are geometric isomers. Note their shapes in both ball-andstick models and spacefilling models. 䊳
CH3
H
cis-2-butene
trans -2- butene 18.1
Hydrocarbons
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BIOLOGY
CONNECTION
Vision and Vitamin A Vision is a process usually studied in biology class. That’s where you may have learned that light rays pass through the eye to reach the retina, where the rods and cones are located. Rods and cones are nerve receptors that are excited by light. More than 120 million rods in each eye detect white light and provide sharpness of visual images. The 7 million cones in each eye detect color. The pigment molecules responsible for vision are attached to the ends of the rods and cones. One of these pigments is called rhodopsin. Rhodopsin has two parts—a protein called opsin and a small molecule called retinal. Chemistry of vision Beta-carotene, the natural orange pigment found in carrots and other yellow or green vegetables, breaks down in your body to form vitamin A, which is then converted to 11-cis-retinal. In the retina of the eye, 11-cis-retinal attaches to the protein opsin to form rhodopsin. When light is absorbed by the 11-cis isomer portion of rhodopsin, the energy causes the pigment molecule to undergo a change in shape. The right end of the molecule rotates about a double bond to form all-trans-retinal, in which all the groups are in the trans position. H H H H
H
CH3
C
H H
H
CH3
C
C
C
H
CH 3
C
C
C
CH3 CH3 H
H
H
H
11-trans - retinal
H H H H
H C
H H
H
CH3
H
C
C C
C
CH3 CH3 H
H
C H
C C
H3C
C 11-cis - retinal
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Organic Chemistry
Night vision When large quantities of light energy strike the rods, large amounts of rhodopsin are broken down. As a consequence, the concentration of rhodopsin in the rods falls to a low level. If you leave a bright area and enter a darkened room, the quantity of rhodopsin in the rods at first is small. As a result, you experience temporary blindness. The concentration of rhodopsin gradually builds up until it becomes high enough for even a small amount of light to stimulate the rods. During dark adaptation, the sensitivity of the retina can increase as much as a thousandfold in only a few minutes and as much as 100 000 times in an hour or more.
C
C
CH3
When the retinal portion of rhodopsin is isomerized to the all-trans form, it separates from the opsin. Thus, light energy causes the rhodopsin to break down into the substances from which it was formed. As the rhodopsin molecule splits, the rods become excited, probably due to ionic charges that develop on the splitting surfaces. These charges last only for a second, but they generate nerve signals that are transmitted to the optic nerve and then to the brain. After the rhodopsin is activated, the trans-retinal returns to the cis form and recombines with opsin. This process is relatively slow, which is why your eyes need time to adjust to dim light.
O
O
Connecting to Chemistry 1. Hypothesizing Owls and bats avoid daylight and are at home in the dark. Their eyes contain only rods. Hypothesize as to the relative amount of rhodopsin in their
rods compared with that in human eyes, and explain. 2. Analyzing What is the significance of the change from 11-cis-retinal to alltrans-retinal for the vision process?
䊴 When corn oil is hydrogenated, it becomes solid. This is how margarine is produced and explains why margarine is solid at room temperature, whereas the vegetable oil from which it is derived is liquid.
Figure 18.8 Saturated and Unsaturated Fats 䊱 Most vegetable oils contain unsaturated alkenes, whereas butter and lard—both animal products—contain mostly saturated alkanes. Because alkanes have higher melting points than alkenes, fats from animals are solid at room temperature. Fats from plants generally are liquids.
Alkenes are more reactive than alkanes because the two extra electrons in the double bond are not held as tightly to the carbons as are the electrons in a single bond. Alkenes readily undergo synthesis reactions in which smaller molecules or ions bond to the atoms on either side of the double bond. An unsaturated alkene can be converted into a saturated alkane by adding hydrogen to the double bond. This reaction is called hydrogenation. Its effects are shown in Figure 18.8. CH2=CH2 ⫹ H2 ˇ CH3CH3 Ethene Ethane (unsaturated) (saturated)
Alkynes Another type of unsaturated hydrocarbon, called an alkyne, contains a triple bond between two carbon atoms. Alkynes are named using the alkane root name for a given carbon chain length and changing the -ane ending to -yne. Ethyne, known more commonly as acetylene, is the most important commercial alkyne. Most acetylene produced in the United States is used to make vinyl and acrylic materials, although about ten percent is burned in oxyacetylene torches. These torches are used to cut and weld metals. Few alkynes are known to occur naturally because they are very reactive. However, they can be synthesized from other organic compounds. The names and structures of some small alkyne molecules are shown in Table 18.2. Table 18.2 Simple Alkynes Chemical Name ethyne propyne 1-butyne 2-butyne
Common Name acetylene methylacetylene — —
Structure HC≠CH HC≠CCH3 HC≠CCH2CH3 CH3C≠CCH3
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Hydrocarbons
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Meet John Garcia, Pharmacist “A treasure house going up in smoke”—that’s how pharmacist John Garcia describes the fires set to clear land in the Amazon. He’s concerned about the potential medicines that humans might be destroying as they make inroads into formerly wild areas. “However,” he continues, “endangered sources for new medicines aren’t only in jungles. Lincomycin, an antibiotic, was discovered in soil in Lincoln, Nebraska.” In this interview, Mr. Garcia describes some of the changes he has seen in his four decades as a pharmacist.
On the Job Mr. Garcia, can you tell us what you’ll be doing first today in your pharmacy? This morning, I have to put together 100 suppositories for a patient with migraine headaches. At least that won’t be as difficult as the order for a suppository a colleague of mine once had. That was for an elephant! The medication was made with an aluminum baseball bat mold. Just one dose cost $300. Do you prepare prescriptions for other animals? I’ve made them for rabbits. I added raspberry flavor because rabbits enjoy the flavor. Do your human patients get a choice of flavors, too? I’ve got about 40 flavors—piña colada, bubble gum, peppermint. Tutti-frutti is my personal favorite. These flavorings aren’t
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just frills. For instance, I prepare chloroquine, an antimalarial drug for children, and put it in a chocolate base. Because the molecules of chocolate are fairly large, they block the taste buds and cover up the bitterness of the medicine. You own and operate your own independent pharmacy. How does it differ from most pharmacies? This one is different because I specialize in compounding prescriptions. That means that I can prepare customized formulas in precise doses to meet patients’ individual needs. In most pharmacies, the pharmacists do mostly what I call “count and pour.” That is, they prepare standardized doses of drugs. A lot of times, I work in partnership with a patient’s doctor. He or she will ask my advice about medication. Besides suggesting alternative drugs or doses, I can talk with the doctor about methods of administering a drug. How has the practice of pharmacy changed in the 40 years you’ve been a pharmacist?
Were you, as a child, aware of pharmacies and medications? Not really. I don’t think I even saw a doctor until I needed a physical exam to play high school sports. My parents are from Mexico, and my mother knew many home remedies. She gave me things like charcoal or mint for an upset stomach.
Personal Insights New laws have given pharmacists the responsibility of explaining carefully to a patient the workings of a drug and its possible side effects. At the same time, changes in the insurance industry have added lots of paperwork and reduced our fees. So that means that pharmacists have to work more quickly while maintaining careful standards. Seriously ill patients are living longer today. How has that change affected your practice? I work with several hospices, which care for terminally ill patients. By the time patients are admitted to a hospice, they typically are in the last stages of an illness and have about 40 days to live. Our priority is to make the patient comfortable by using a variety of medications to relieve pain and discomfort.
Early Influences
What qualities do you think it takes to be a good pharmacist? It’s obvious that you have to like people and be accurate in mathematical and chemical calculations. I also like to say that pharmacy is a lot like cooking—a little of this, a little of that. Just like a cook, I want my preparations to contain the right proportion of ingredients, to taste good, and also to look elegant. What changes do you see coming in the field of pharmacy during the 21st century? I think people will be able to insert a prescription in something like an automated teller machine. They’ll also insert medical cards encoded with their personal medical history and their age, height, and weight. The machine will dispense their medication.
CAREER
CONNECTION
How did you come to be a pharmacist? These careers are all related to the field of pharmacy. I got an after-school job in a small pharmacy when I was in high school. I started out making deliveries and eventually helped fill prescriptions. I liked working with the public, and the pharmacist, Nathan Fisher, encouraged me to apply to pharmacy school. I feel honored that he was my mentor.
Pharmacologist Four to six years post-college study at a medical school or school of pharmacy Pharmaceutical Technician Two years in a training program after high school Pharmaceutical Production Worker High school diploma and on-the-job training
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Hydrocarbons
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Melting and boiling points of alkynes increase with increasing chain length, just as they did for alkanes and alkenes. Alkynes have physical and chemical properties similar to those of alkenes; their melting points are higher than those of alkanes, and they undergo synthesis reactions. For example, hydrogen molecules can be added to an alkyne in a stepwise fashion to form an alkene and then an alkane. CH≠CH ⫹ H2 ˇ CH2=CH2 CH2=CH2 ⫹ H2 ˇ CH3CH3
August Kekulé (18291896) was a German student of architecture who became interested in chemistry. One of the biggest puzzles of 19th-century chemistry involved the structure of benzene, which seemed to have too few hydrogen atoms to exist as a straight-chain molecule. Kekulé solved the puzzle after having a dream in which a dancing snake formed a ring and bit its own tail. When he awoke, he worked out the ring structure of benzene. Kekulé’s work is considered to be among the most important in 19th-century theoretical organic chemistry.
Aromatic Hydrocarbons Another group of unsaturated hydrocarbons has distinctive, six-carbon ring structures. The simplest compound in this group is benzene, with the molecular formula C6H6. Benzene contains six carbons joined together in a flat ring. Its structure was originally thought to contain alternating single and double bonds between the carbons, as seen in Figure 18.9, but this structure is now known to be incorrect. Although it contains double bonds, benzene does not share most of the properties of alkenes. It is unusually resistant to hydrogenation, whereas most alkenes readily become hydrogenated. To account for this inertness, chemists have suggested that the extra electrons are shared equally by all six carbons in the ring rather than being located between specific carbon atoms. The currently accepted structure for the benzene molecule is shown in Figure 18.9. This sharing of electrons among so many carbons gives benzene and similar compounds unique properties. The name aromatic hydrocarbon is used to describe a compound that has a benzene ring or the type of bonding exhibited by benzene. Aromatic hydrocarbons were originally named because most of them have distinctive aromas. Naphthalene, formerly used in mothballs to prevent moth damage to woolen clothes,
Figure 18.9 The Structure of Benzene The structure of benzene can be represented in different ways.
The flat benzene molecule is shown with clouds of shared electrons above and below the plane of the ring. 䊲
H H
H
H
H H
䊱 This structure of benzene, showing alternating single and double bonds, is now known to be incorrect because it does not account for benzene’s inertness.
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The hexagon diagram is a more accurate shorthand representation. In this hexagon, each corner represents a carbon atom. The circle in the middle of the structure represents the cloud of six electrons that are shared equally by the six carbon atoms in the molecule. 䊲
consists of two benzene rings attached side-by-side. Aromatic hydrocarbons are unusually stable because the carbons are bonded tightly together by so many electrons.
Sources of Organic Compounds
petroleum: petra (L) a rock oleum (L) oil Petroleum is an oily fossil fuel found naturally in rock strata of certain geological formations.
You have examined structures and learned how to name basic organic compounds, but have you stopped to think about where these compounds come from? Most hydrocarbons come from fossil fuels, especially petroleum, but also natural gas and coal, Figure 18.10. Other important sources include wood and the fermentation products of plant materials. Natural gas contains large quantities of methane, along with smaller amounts of alkanes up to about five carbons in length. Natural gas passes easily through pipes and is used mostly as a fuel, but it also serves as a raw material for making many small organic compounds. Petroleum is a complex mixture, mostly of alkanes and cyclic alkanes. Products we get from petroleum include gasoline, jet fuel, kerosene, diesel fuel, fuel oil, asphalt, and lubricating oil. To use these organic products, they have to be separated from one another. What properties of hydrocarbons might allow them to be separated? Figure 18.10 Sources of Aromatic Hydrocarbons Soot is full of aromatic hydrocarbons produced by combustion of organic materials such as wood or coal. In 1775, a British physician investigating the high incidence of cancer among chimney sweeps theorized that soot in the chimneys was the cause. In the 1930s, it was shown that a number of large hydrocarbons with ring structures found in the coal tar in soot were carcinogenic, or cancer-causing. 䊲
䊱 Valuable deposits of petroleum and natural gas often are found by offshore oil drilling. These deposits are common under the oceans because microscopic marine organisms—including algae, bacteria, and plankton—died and settled on the seafloor where their remains were altered by high temperatures and pressure. Petroleum deposits also are found under dry land because at one time, this land was under a sea.
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Hydrocarbons
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Figure 18.11 Fractional Distillation of Petroleum To separate important components of petroleum using fractional distillation, petroleum or hot crude oil is heated in a furnace. The liquid alkanes are vaporized and allowed to rise in the fractionating tower. Gases that were dissolved in the petroleum are removed at the top of the tower and condensed into liquids that are sold in cylinders. Gasoline is part of the next-lower group of materials drawn off. Below the gasoline fraction come mixtures of heavier hydrocarbons, such as kerosene, fuel and lubricating oils, and asphalt.
Volatile gases
Gasoline Kerosene Heating oil
Furnace Lubricating oil Asphalt 700°F Steam Crude oil
distillation: de (L) down stillare (L) to drop Distillation is a process by which one compound is removed from others drop by drop as it evaporates out of a solution.
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Residue
One property is boiling point. As you learned in Chapter 5, distillation is a technique used to separate substances that have different boiling points. In the petroleum industry, huge towers are used to distill petroleum into its component liquids. Inside the tower, many plates provide multiple surfaces on which repeated vaporization-condensation cycles take place. Repeated cycles provide for more efficient separations and allow fractions containing only one or a few different compounds to be isolated. This method of separation is called fractional distillation. Figure 18.11 shows where different products are drawn off according to boiling points at different levels of the tower. The temperature of the tower is controlled so that it is hotter at the bottom and cooler at the top. Because boiling point increases along with molecular weight, the heavier, higher-boiling components condense near the bottom, and the lighter, lower-boiling molecules condense toward the top. Pipes connected at various points allow chemical engineers to draw off each fraction. The process of fractional distillation does not produce quantities of hydrocarbons in the proportions needed in industry. For example, gasoline is usually the fraction of petroleum most in demand, but this fraction makes up less than half of petroleum. How can the quantities of useful hydrocarbons be increased? The hydrocarbons in the fractions coming off the lower parts of the tower are mostly large alkanes. These can be converted into smaller, more useful alkanes and alkenes by a process called cracking. Cracking uses catalysts or high temperatures in the absence of air to break down or rearrange large hydrocarbons. Cracking is also used to increase the yield of natural gas by producing small alkenes from larger molecules. The cracking of propane produces some methane and ethene, in addition to propene and hydrogen. ˇ CH4 ⫹ CH2=CH2 ⫹ CH3CH=CH2 ⫹ H2 2CH3CH2CH3 500–700°C Organic Chemistry
Underground coal mining can be difficult, dirty, and dangerous work. The coal seams are often found deep underground, and tunnels must be dug so that miners can reach them. A mining machine breaks up the coal and delivers it to a conveyor belt, which carries the coal chunks out of the mine. 䊲
Figure 18.12 Sources of Coal 䊱 Coal is our most plentiful fossil fuel. The most inexpensive way to obtain coal that is not deeply buried is by strip-mining, in which large areas of land are stripped bare of all vegetation. This causes environmental problems when the exposed soil washes away after the coal is removed. Laws in the United States now require that most stripped areas be restored.
Another process that uses heat, pressure, and catalysts to convert large alkanes into other compounds is called reforming. It is used to form aromatic hydrocarbons. Like petroleum and natural gas, coal also is a fossil fuel. It is formed from the remains of plants that became buried underwater and were subjected to increasing pressures as layers of mud built up. Coal is composed primarily of carbon but also contains many mineral impurities. It is used mostly as a fuel and as a source of aromatic hydrocarbons. Coal must be obtained from underground or surface mines, as shown in Figure 18.12.
SECTION REVIEW Understanding Concepts 1. Name the organic compounds shown. a) CH3CH=CHCH2CH2CH3 CH3 b) CH3CHCH2CH2CH3 c) CH3CH2CH2CH2CH2CH2CH3 CH3 CH3 d) CH3CH2CHCH2CHCH3 2. Draw structures that correspond to the names of the hydrocarbons given. a) hexane b) 3-ethyloctane c) trans-2-pentene chemistryca.com/self_check_quiz
d) ethyne e) 1,2-dimethylcyclopropane f) 1-butyne 3. What are the major sources of hydrocarbons? Where are they found?
Thinking Critically 4. Interpreting Chemical Structures Halogen molecules such as Br2 can be added to double bonds in a reaction similar to hydrogenation. Draw the structure of the product that forms when Br2 is added to propene.
Applying Chemistry 5. Storage of Hydrocarbons Would it be more important to store octane or pentane in a tightly sealed bottle at a low temperature? Why? 18.1
Hydrocarbons
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Substituted Hydrocarbons
SECTION
18.2
D
oesn’t that apple pie look good, fresh out of the oven? Looking at the photo, you can almost smell the apples, cinnamon, nutmeg, and perhaps vanilla. What causes these ingredients to have aromas? Apples, cinnamon, nutmeg, vanilla, and many other fruits and spices all contain molecules with a distinctive group of atoms located at the end of the molecule. This atomic arrangement imparts a pleasant odor to the molecule.
SECTION PREVIEW Objectives ✓ Compare and contrast the structures of the major classes of substituted hydrocarbons. ✓ Summarize properties and uses of each class of substituted hydrocarbon.
Functional Groups
Review Vocabulary
When chlorine and methane gases are mixed in the presence of heat or ultraviolet light, an explosive reaction takes place in which a mixture of the following four products is produced. Because structures of these compounds are the same as hydrocarbons except for the substitution of atoms of another element for part of the hydrocarbon, they are called substituted hydrocarbons. The part of a molecule having a specific arrangement of atoms that is largely responsible for the chemical behavior of the parent molecule is called a functional group. Functional groups can be atoms, groups of atoms, or bond arrangements. Notice that in the structures shown, one or more of the hydrogen atoms in methane is replaced by a chlorine atom.
Alkane: a saturated hydrocarbon that contains only single bonds between carbon atoms.
New Vocabulary substituted hydrocarbon functional group
Cl H
C
Cl H
H
H Chloromethane (methyl chloride)
C
Cl Cl
H Dichloromethane (methylene chloride)
Cl
C
Cl Cl
H Trichloromethane (chloroform)
Cl
C
Cl
Cl Tetrachloromethane (carbon tetrachloride)
Replacing part of a hydrocarbon with functional groups changes the structure, properties, and uses that we have for the compounds. For example, chloromethane, a gas, is used as a refrigerant, and dichloromethane, a liquid, is used as a solvent to decaffeinate coffee. You may know trichloromethane by its common name, chloroform. It was an early anesthetic, used to put people to sleep during surgery. Tetrachloromethane, commonly called carbon tetrachloride, is a solvent that was used in dry cleaning and in fire extinguishers. 640
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Some functional groups are complex in structure and consist of a group of atoms rather than a single atom. These groups of atoms often contain oxygen or nitrogen, and some contain sulfur or phosphorus. Double and triple bonds also are considered to be functional groups. Many organic compounds contain more than one type of functional group. Simple organic compounds are grouped into categories depending on the functional group they contain. You will study structures and examples of the most important functional groups.
Functional Groups: Structure and Function As you study these functional groups, notice that the similarity in properties among molecules containing a given functional group leads to the use of members of that group for similar purposes. In the structures shown below, R and R′ each represent a hydrocarbon part of the molecule. R and R′ may be the same, or they may be different. For example, in the formula for an ether, R—O—R′, where R represents CH3CH2— and R′ represents CH3, the ether has the formula CH3CH2OCH3. 1 Halogenated Compounds structure: R—X , where X ⫽ F, Cl, Br, or I functional group: halogen atoms properties: high density uses: refrigerants, solvents, pesticides, moth repellents, some plastics biological functions: thyroid hormones examples: chloroform, dichloromethane, thyroxin, Freon, DDT, PCBs, PVC
F Cl
C
Cl
F Freon
Chlorofluorocarbons (CFCs) are substituted compounds containing chlorine and fluorine atoms bonded to carbon. The most common CFC, Freon, has the formula CCl2F2. CFCs were once widely used as propellents in aerosol cans; as solvents; as the foaming agent in the manufacture of plastic foam materials; and as refrigerants in air conditioners, refrigerators, and freezers. However, in 1987, the major industrial nations of the world agreed to a gradual reduction in the use of CFCs because they cause a depletion of the valuable ozone in Earth’s upper atmosphere. CFCs are being replaced by other halogenated compounds that are not as damaging to the atmosphere. 18.2
Substituted Hydrocarbons
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H
H
H
C
C
2 Alcohols structure: R—O—H
OH
functional group: hydroxyl group (—OH); hydrogen atom bonded to an oxygen atom, which is bonded to the hydrocarbon part of the molecule properties: polar, so water molecules are attracted to it; high boiling point; alcohols with low molecular weights are water-soluble uses: solvents, disinfectants, mouthwash and hair-spray ingredient, antifreeze biological functions: reactive groups in carbohydrates, product of fermentation examples: methanol, ethanol, isopropanol (one type of rubbing alcohol), cholesterol, sugars
H H Ethanol
Lab See page 871 in Appendix F for Comparing Water and Alcohol
An organic compound that contains at least one hydroxyl group is called an alcohol. The name of an alcohol ends in -ol. Alcohols have many different uses. One of the most important is as a disinfectant for killing bacteria and other potentially harmful microorganisms. For this reason, ethanol is put into mouthwashes, and rubbing alcohol is used as a disinfectant. Antifreeze also is an alcohol.
3 Carboxylic Acids O structure: R
C
O
H
functional group: carboxyl group (—COOH); oxygen atom doublebonded to a carbon, which is also bonded to a hydroxyl group and the hydrocarbon part of the molecule properties: acidic, usually water-soluble, strong unpleasant odors, form metal salts in acid-base reactions uses: vinegar, tart flavoring, skin-care products, production of soaps and detergents biological functions: pheromones, antsting toxin; causes rancid-butter and smelly-feet odors examples: acetic acid (in vinegar), formic H O acid, citric acid (in lemons), salicylic acid
H A compound containing a carboxyl group is known as a carboxylic acid, or organic acid. Many pheromones contain carboxyl functional groups. Pheromones are organic compounds used by animals to communicate with each other. When an ant finds food, it leaves behind a pheromone trail that other ants in its colony can follow to get to the food source.
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Organic Chemistry
C
C
O
H Acetic acid
H
4 Esters O structure: R
C
O
R'
derived from carboxylic acids in which the —OH of the carboxyl group has been replaced with an —OR from an alcohol properties: strong aromas, volatile uses: artificial flavorings and fragrances, polyester fabric biological functions: fat storage in cells, DNA phosphate-sugar backbone, natural flavors and fragrances, beeswax examples: banana oil, oil of wintergreen, triglycerides (fats) A compound formed from the reaction of an organic acid and an alcohol is called an ester. Some esters are used in the production of polyester fabrics. Many esters are used as flavorings in food products. Natural flavors are often complex mixtures of esters and other compounds, whereas artificial flavors usually contain fewer compounds and may not taste exactly the same as the natural flavor.
H
H
H
H
O
C
C
C
C
H
H
H
O
H
H
C
C
H
H
H
Ethyl butyrate (pineapple flavoring)
18.2
Substituted Hydrocarbons
643
5 Ethers structure: R—O—R′; oxygen atom bonded to two hydrocarbon groups properties: mostly unreactive, insoluble in water, volatile uses: anesthetics, solvents for fats and waxes examples: diethyl ether
H
H
H
C
C
H
H
O
H
H
C
C
H
H
H
Ethyl ether An ether is an organic compound in which an oxygen atom is bonded to two hydrocarbon parts of the molecule. Diethyl ether, often simply called ether, is an effective anesthetic. Because it is insoluble in water, it passes readily through the membranes surrounding cells. Ether is rarely used as an anesthetic today because it is highly flammable and because it causes nausea.
O 6 Ketones and Aldehydes O structures: ketones: R
H C
C
H
O aldehydes: R
R'
C
O
H
OH functional group: carbonyl group (—CO); carbon atom double bonded to an oxygen atom properties: very reactive, distinctive odors uses: solvents, flavorings, manufacture of plastics and adhesives, embalming agent examples: acetone; formaldehyde; cinnamon, vanilla, and almond flavorings
H
H
O
H
C
C
C
H
H Acetone
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C H
Vanillin
H Aldehydes and ketones are compounds that contain a carbonyl group. If the carbonyl group is on the end of the carbon chain, the compound is called an aldehyde. If the carbonyl group is not at the end of the chain, the compound is a ketone. Acetone is a ketone solvent commonly used in nailpolish remover. Nail polish is not soluble in water— if it were, it would come off when you wash your hands. It is soluble in many organic compounds such as acetone, which is used to remove it.
H
7 Amines and Amides O structure: amine: R—NH2
amide: R
C
NH2
functional group: amine: amino group (—NH2); two hydrogen atoms bonded to a nitrogen atom, which is bonded to the hydrocarbon part of the molecule amide: amino group bonded to a carbonyl group (—CONH2) properties: amines: basic, ammonia-like odor amides: neutral, most are solids uses: solvents, synthetic peptide hormones, fertilizer, nylon synthesis biological functions: in amino acids, peptide hormones, and proteins; distinctive odor of some cheeses examples: urea, putrescine, cadaverine, Nutrasweet
H N
H
H
H
H
H
C
C
C
C
H
H
H
H
H N H
Putrescine
An organic compound containing an amino group is called an amine. Amines and amides are important biological molecules because they are part of proteins. When an organism dies, its proteins are broken down into many different compounds containing amino functional groups. Given that two of those compounds are named putrescine and cadaverine, what type of odor do you think they have? These compounds have a distinctive, unpleasant smell that specially trained sniffer dogs can use to locate human remains and to help in forensic investigations. Cadaverine also contributes to bad breath.
18.2
Substituted Hydrocarbons
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A Synthetic Aroma When brought together under the proper conditions, an organic acid and an alcohol will react to form an ester. Esters are generally volatile compounds having distinctive odors. In this MiniLab, you will combine methanol and salicylic acid to produce the ester, methyl salicylate.
2
Procedure
Plants are a common source of esters.
1. Put on an apron and goggles. 2. Set up a hot-water bath for the reaction by filling a 250- or 400-mL beaker about halfway with tap water and setting it on a hot plate or on a wire gauze, iron ring, and ring stand over a burner flame. Allow the water to warm but not boil. 3. Place 3 mL of methyl alcohol and 1 g of salicylic acid in a large (20 mm ⫻ 150 mm) test tube. Mix the reactants with a glass stirring rod. 4. Add about 0.5 mL (about ten drops from a dropper) of concentrated sulfuric acid to the reaction mixture and stir. Do not draw the acid into the rubber bulb of the dropper. (CAUTION: Concentrated sulfuric acid is corrosive to skin, eyes, body tissues, and most other organic materials. If any contact occurs, notify your teacher immediately.)
5. Place the test tube and contents in the hot-water bath and allow it to warm for five or six minutes. 6. Pour the contents of the test tube into about 50 mL of cold, distilled water in a small beaker. Cover the beaker with a watch glass and allow it to stand for a minute or two. 7. Remove the watch glass and waft the methyl salicylate odor toward your nose. Record your observations. Analysis 1. What familiar odor is caused by methyl salicylate? 2. Look up the structural formulas for the reactants in a reference book, and write the equation for the reaction using structural formulas for all organic reactants and products. 3. What organic reactants would be required to produce the ester, ethyl butyrate?
The Sources of Functional Groups How can different compounds containing functional groups be made? Alkanes are not very reactive, so it can be difficult to react them directly to form substituted molecules. You read earlier of one reaction used to substitute chlorine atoms for hydrogen atoms in methane. However, that reaction is not practical because a mixture of four different products is formed. Each must be separated and purified before it can be used. Alcohols are a better choice than alkanes as the raw materials for synthesizing organic molecules. Alcohols can be converted into compounds containing almost every other type of functional group. For example, an acid and an alcohol react together to form an ester and water. 646
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Figure 18.13 Production of Ethanol Ethanol can be produced industrially by the fermentation of sugars and starches by yeast cells. These sugars and starches come from plant material such as corn, molasses from sugarcane, and the grapes grown in vineyards like this one.
Where do all the alcohols needed by industry come from? Cracking of petroleum products produces alkenes, which can readily be converted into alcohols by reaction with water. This process is used to synthesize ethanol, the two-carbon compound that contains an alcohol functional group. H
H C
⫹
C
ˇ
H2O
H
H
H
⫹
Ethene
ˇ
Water
H
OH
C
C
H
H H Ethanol
A second important natural source of ethanol provides a convenient supply of this reactive molecule. Ethanol is produced by yeast cells when they ferment the sugars and starches in plant materials, as discussed in Figure 18.13. Fermentation produces much smaller quantities of ethanol than does the reaction between ethene and water; it is used in industry mainly to produce the ethanol in alcoholic beverages. For more practice with solving problems, see Supplemental Practice Problems, Appendix B.
SECTION REVIEW Understanding Concepts
Applying Chemistry
1. Explain why organic compounds with similar structures often have similar uses. 2. Distinguish between an aldehyde and a ketone. 3. Redraw the following structure of thyroxine, a thyroid hormone. Circle and name each type of functional group present. I I
5. Antifreeze Ethylene glycol, used as an antifreeze in car radiators, has two hydroxyl groups, as shown below. What can you infer about the boiling point, freezing point, and solubility in water of this compound from its structure? OH OH
O
HO I
CH2CHCOOH I
H
NH2
C
C
H
H
H
Thinking Critically 4. Inferring Explain why alkenes and alkynes contain functional groups but are not substituted hydrocarbons. chemistryca.com/self_check_quiz
18.2
Substituted Hydrocarbons
647
SECTION
18.3 SECTION PREVIEW Objectives ✓ Identify the monomers that form specific polymers, and draw structural formulas for polymers made from given monomers. ✓ Differentiate between condensation and addition polymerization reactions. ✓ Summarize the relationship between structure and properties of polymers.
Review Vocabulary Functional group: the part of a molecule that is largely responsible for the chemical behavior of the molecule.
New Vocabulary polymer monomer addition reaction condensation reaction cross-linking thermoplastic thermosetting
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Plastics and Other Polymers
O
n April 6, 1938, a young chemist working on the preparation of a compound used in refrigeration opened the valve on a tank of the reactant he planned to use in this process, tetrafluoroethene gas. Roy J. Plunkett was puzzled when no gas came out after he opened the valve because the tank was heavy enough to indicate that it was full of gas. He poked a wire through the opening of the valve on the tank and found that it wasn’t clogged. What do you think you would have done if you had been in Plunkett’s shoes? Would you have discarded the tank in favor of a new one? Or would you have seen this problem as an intriguing puzzle to be investigated? Plunkett was curious, and when he cut open the tank, he discovered a white, waxy solid in place of the gas he expected. Today, this solid is known as Teflon, a large molecule with remarkable properties. You are probably familiar with Teflon as a nonstick coating on pots and pans, but it is also used for dentures, artificial joints and heart valves, space suits, and fuel tanks on space vehicles.
Monomers and Polymers How did a white, waxy solid form from the colorless gas tetrafluoroethene (also called tetrafluoroethylene)? One clue to what type of reaction occurred can be found in the properties of those two substances. Most nonpolar molecules that are relatively small tend to be gases at room temperature, whereas larger molecules usually are solids. When the structure of Teflon was determined, the molecule was found to consist of chains of carbons with fluorine atoms attached. Somehow, the tetrafluoroethylene molecules in the gas had reacted with each other to form these long-chain molecules. A large molecule that is made up of many smaller, repeating Organic Chemistry
Figure 18.14
F
F C
F
Teflon Teflon provides a good coating for this skillet because it is unreactive and food will not stick to it.
C F
Tetrafluoroethylene
F
F
F
F
F
F
C
C
C
C
C
C
F
F F F F Teflon polymer
F
units is called a polymer. A polymer forms when hundreds or thousands of these small individual units, which are called monomers, bond together in chains. The monomers that bond together to form a polymer may all be alike, or they may be different. When Teflon formed in the tank, many small tetrafluoroethylene monomers combined to form long polymers of polytetrafluoroethylene. Examine the structures in Figure 18.14 to help you visualize this reaction. The properties of a polymer are different from those of the monomers that formed it. For example, the polyethylene plastic in milk jugs is made when molecules of gaseous ethylene react to form long chains. The unique properties of a given polymer, such as tensile strength, waterrepellency, or flexibility, are related to the polymer’s enormous size and the way its monomers join together.
Synthetic Polymers Polymers are everywhere, making fabrics such as nylon and polyester, plastic wrap and bottles, rubber bands, and many more products you see every day. How many polymers can you identify in Figure 18.15? What do the polymers that make up such different substances have in common? All are large molecules made of smaller, repeating units. Chemists have been synthesizing polymers in laboratories for only about 100 years. Can you imagine what life was like before people had these synthetic polymers? You might have gotten wet on the way to school without a nylon raincoat, eaten a stale sandwich for lunch without plastic wrap or a container to put it in, and worn a heavier cotton uniform in your sports activity instead of a lightweight, synthetic fabric. Many polymers have added conveniences that we take for granted in our lives.
polymer: poly (GK) many meros (GK) part Polymers are large molecules made of many smaller parts.
Figure 18.15 Synthetic Polymers Sports activities would be different today without synthetic polymers. Balls, uniforms, artificial turf, bandages used to wrap sprains, and nets used in hoops and goals are usually made of synthetic polymers. 18.3
Plastics and Other Polymers
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Identification of Textile Polymers For centuries, polymers have been used for clothing in the form of cotton, wool, and silk. Chemists trying to improve these natural polymers to fit specific purposes have designed many synthetic polymers that also are used to make fabrics. Among these are nylon, rayon, acetate, and the polyesters. Many synthetic fabrics are designed to look and feel like natural polymers but also have the superior properties for which they were designed, such as being wrinkle-resistant, water-repellent, or quick-drying. However, these imitators can’t fool a chemist; simple tests that can be done in minutes will distinguish the real thing from a pretender. This is because differences in their structures lead to differences in properties. In this experiment, you will identify fabric samples by testing them for characteristic properties. Problem What tests can be performed that will differentiate among the polymers used to make fabrics? Objectives Analyze changes in fabric samples that are subjected to flame and chemical tests. Classify fabrics by polymer type based on test results.
• •
Materials fabric samples A-G (7 types; six 0.5 ⫻ 0.5 cm squares of each type) Bunsen burner
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Chapter 18
beaker containing water forceps test-tube holder test-tube rack
Organic Chemistry
medium test tubes (4) balance watch glass red litmus paper stirring rod 100-mL beakers (2) 10-mL pipet 25-mL graduated cylinder
Ca(OH)2 1M BaCl2 concentrated H2SO4 iodine solution 0.05M CuSO4 3M NaOH acetone
Safety Precautions Use care when working with an open flame and handling concentrated acids and bases. Do not inhale odors from plastics.
Flame Tests 1. Use forceps to hold a square of fabric A in a Bunsen burner flame for 2 seconds. 2. Remove the fabric from the flame, and blow out the flame if the fabric keeps burning. 3. Observe the odor by wafting smoke from the smoldering sample toward your nose. Be sure the fabric is no longer burning by immersing it in a beaker of water. 4. Make a data table similar to Table 1 in Data and Observations on page 652. Record your observations about the way the fabric burns in the flame, odor observed, and characteristics of the residue left after burning. 5. Repeat steps 1 to 4 six times, using fabric samples B-G. 6. Use the following table to make a preliminary identification of your samples.
Polymer Type
Type of Burning
Burning Odor
Residue Type
Silk or wool
Burns and chars
Hair
Crushable bead
Cotton
Burns and chars
Paper
Ash
Nylon, polyester, acetate, or acrylic
Burns and melts
Chemical
Plastic bead
Chemical Tests Use your preliminary identifications to determine which tests are necessary to identify each fabric sample. You should not have to carry out every test on every sample. Make a data table similar to Table 2 on page 652, and record all your results. 7. Nitrogen Test Place a fabric square in a test tube and add 1 g of Ca(OH)2. Using a testtube holder, heat the tube gently in a Bunsen burner flame while holding a piece of red litmus paper with forceps over the mouth of the tube. If the litmus paper turns blue, nitrogen is present. Only silk, the contents to another beaker containing ten wool, nylon, and acrylic contain nitrogen. drops of iodine solution in 25 mL of water. Rinse the empty beaker with large quantities of water. 8. Sulfur Test Add a fabric square to 10 mL of Cotton gives a dark blue color within 1 to 2 min3M NaOH in a test tube, and gently heat to utes, and acetate gives this color after 1 to 2 boiling by holding the tube in a Bunsen burnhours. (CAUTION: Handle the beaker containing er flame. Be careful that the tube is not pointsulfuric acid with care.) ing toward anyone. Cool the solution, add 30 drops of BaCl2 solution, and observe whether 10. Protein Test Place a fabric square on a watch or not a precipitate forms. Only wool conglass, and add ten drops of 0.05M CuSO4. Wait tains enough sulfur to give a barium sulfide 5 minutes, and then use forceps to dip the fabric precipitate. into 3M NaOH in a test tube for 5 seconds. Silk and wool are protein polymers, and a dark violet 9. Cellulose Test Place a fabric square in a color will appear on those fabrics after doing beaker, cover with approximately 2 mL of conthis test. centrated H2SO4, and then carefully transfer
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11. Formic Acid Test Place any sample to be subjected to the formic acid test in a test tube and bring it to your teacher, who will perform this test in the fume hood. The teacher will add 1 mL of formic acid to the test tube and stir with a glass rod. Note whether or not the fabric dissolves in the solution. Silk, acetate, and nylon dissolve in formic acid. 12. Acetone Test Add a fabric square to 1 mL of acetone in a test tube, stir with a glass rod, and note whether or not the fabric dissolves. Only acetate dissolves in acetone. (CAUTION: Be careful to do this test away from any open flames.) 13. Dispose of all products of your tests as directed by your teacher.
1. Thinking Critically Why do you think it was necessary to add NaOH and heat before adding BaCl2 in the sulfur test?
Table 1
2. Comparing and Contrasting Can you make any conclusions about the way that synthetic and natural polymers burn? 3. Classifying Use your data from the flame and chemical tests to classify each fabric sample you tested as a silk, cotton, wool, nylon, acrylic, acetate, or polyester polymer.
1. What does the plastic, beadlike residue left by some polymers after burning tell about the polymers’ structures? 2. If burning silk and wool smell like burning hair, what does this tell about the structure of hair? 3. What results of the flame and chemical tests would you expect to see if you were testing a fabric sample that was a polyester-cotton blend? 4. Cellulose is a major component of wood as well as of cotton. Would you expect wood to also give a dark blue color in the cellulose test?
Flame Test Observations Sample
Type of Burning
Burning Odor
Residue Type
A
Table 2 Sample
Chemical Test Observations Nitrogen
Sulfur
A
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Cellulose
Protein
Formic Acid
Acetone
When Polymers Meet Water Sometimes, the ability of certain polymers to repel water is useful— this property is what keeps you dry when you wear a raincoat. Other times, it is desirable for polymers to absorb water. This is why wool socks keep your feet warm by wicking water away from your skin and why a diaper helps keep a baby dry. Cloth diapers are made of cotton, a natural polymer that absorbs water well. Why do disposable diapers hold so much more water than cloth diapers? They contain a superabsorbent polymer that can hold hundreds of times its weight in water. In this MiniLab, you will determine how much water the polymers in two different brands of diapers can hold. Procedure 1. Obtain two diapers of approximately the same size but of different brands. 2. Fill a 100-mL graduated cylinder with tap water, and slowly pour the water into the center of one of the diapers. Stop when the water begins leaking out of the diaper. 3. Record the volume of water the diaper holds.
3
4. Repeat steps 2 and 3 with the other diaper. Analysis 1. What could be different about the two diapers that makes one of them hold more water than the other? 2. How might placing a superabsorbent polymer around the roots of a houseplant help it to grow?
Natural Polymers Laboratories are not the only place where polymers are synthesized. Living cells are efficient polymer factories. Proteins, DNA, the chitin exoskeletons of insects, wool, silky spiderwebs and moth cocoons, and the jellylike sacs that surround salamander eggs are polymers that are synthesized naturally. The strong cellulose fibers that give tree trunks enough strength and rigidity to grow hundreds of feet tall are formed from monomers of glucose, which is a sweet crystalline solid. Many synthetic polymers were developed by scientists trying to improve on nature. For example, nylon was developed as a possible silk substitute. The idea for the process by which synthetic threads are formed in factories was borrowed from spiders. Observe in Figure 18.16 the similarities between the spinneret of a spider and an industrial spinneret.
18.3
Figure 18.16 Thread Spinners Long, fine threads are spun when polymer molecules are forced through tiny holes in spinnerets, both natural and industrial.
Plastics and Other Polymers
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Structure of Polymers If you examine the structure of a polymer, you can identify the repeating monomers that formed it. Because polymer molecules are large, they are commonly represented by showing just a piece of the chain. The piece shown must include at least one complete repeating unit. Look carefully at the structure of a segment of a cellulose molecule shown in Figure 18.17. Cellulose is a polymer found in the cell walls of plant cells such as those of wood, cotton, and leaves. It is responsible for giving plants their structural strength. Can you find the portions of the cellulose structure that repeat? Notice that the ring parts of the molecule are all identical. These are the monomer units that combine to form the polymer. Glucose is the name of the monomer found in cellulose. In Figures 18.17 and 18.18, the glucose units are shown in simplified form without carbon and hydrogen atoms. The complete structure of glucose is shown below.
When nylon was first discovered by Wallace Carothers and his colleagues at DuPont in the 1930s, it was not thought to have any useful properties and was set aside on a shelf without even being patented by the company. It wasn’t until some chemists at the company were playing games to see how far they could stretch nylon drawn out into a string that the strength and silky appearance of the nylon threads were noticed. Nylon hosiery was introduced at the 1939 New York World’s Fair in a product display advertised as “Nylon, the Synthetic Silk Made from Coal, Air, and Water!” Four million pairs of nylons were sold in the first few hours after they were offered for sale in New York City on May 15, 1940.
CH2OH O
H HO
H OH
H
H
OH
H OH
Glucose
O O O O O O O O O O Cellulose
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Figure 18.17 Cellulose Cellulose, the main component of plants, is probably the most abundant organic compound on Earth. This plant material, found in the cell walls of fruits and vegetables, cannot be digested by humans. It passes through the digestive tract unchanged, serving as dietary fiber that keeps the human digestive system healthy. Cellulose molecules range in length from several hundred to several thousand glucose units, depending on their source.
Figure 18.18 Starch Starch is digested by humans and serves as an important nutrient. During digestion, starch is broken down into glucose molecules that can be absorbed in the digestive tract. When starchy potatoes and cereal grains are cooked, they form a paste that thickens soups and stews. Starch molecules are usually several hundred glucose units in length. In addition to forming linear chains, glucose molecules form a branched polymer. Starches are a mixture of linear and branched molecules, both consisting only of glucose monomers.
O
O O
O O
O O
O O
O
Starch
Another natural plant polymer that is formed from glucose monomers is starch. Examine the structure of starch in Figure 18.18 to see if you can find the difference between starch and cellulose. Both cellulose and starch are made from only glucose monomers. The difference between them is the way that these monomers are bonded to each other. In starch, the oxygen atom joining each pair of monomers is pointed downward; in cellulose, the oxygen atom is pointed upward. This appears to be only a minor difference, but it changes the properties of the two polymers dramatically.
Polymerization Reactions How do you think the many different types of polymers are formed? Polymerization is a type of chemical reaction in which monomers are linked together one after another to make large chains. The two main types of polymerization reactions are addition polymerization and condensation polymerization. The type of reaction that a monomer undergoes depends upon its structure. 18.3
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H
H C
H
H ⫹
C H
H C
ˇ
C
H
H
H
H
H
H
C
C
C
C
H
Ethylene monomers
Addition ˇ ⫹ ⫹ ⫹ ⫹ Monomers Polymer More ˇ monomers ⫹
H H H Polyethylene
ˇ ˇ
Larger polymer
Figure 18.19 Addition Reactions Ethylene monomers undergo an addition reaction to form the polyethylene that is used in plastic bags, food wrap, and bottles. The extra pair of electrons from the double bond of each ethylene monomer is used to form a new bond to another monomer.
Addition Reactions The reaction by which Teflon is made from its monomer, tetrafluoroethylene, is called an addition reaction. In this type of reaction, monomers that contain double bonds add onto each other, one after another, to form long chains. The product of an addition polymerization reaction contains all of the atoms of the starting monomers. Notice in Figure 18.19 that the ethylene monomer contains a double bond, whereas there are none in polyethylene. When monomers are added onto each other in addition polymerization, the double bonds are broken. Thus, all of the carbons in the main chain of an addition polymer are connected by single bonds. Other polymers made by addition polymerization are illustrated in Figure 18.20.
Figure 18.20 Polymers Made by Addition Polymerization Molten low-density polyethylene can be formed into a film. This tough plastic forms a barrier to food odors, which makes it useful for wrapping and storing foods. 䊲
䊱 Polyvinylacetate is another plastic made by addition polymerization. When mixed with sugar, flavoring, glycerol (for softening), and other ingredients, it becomes chewing gum.
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Chemistry of permanent waves Two kinds of permaChemistry and Permanent Waves Kinds nent-wave lotions are available. Alkaline lotions A series of bad-hair days may drive you to the hair stylist in search of a permanent wave. Your hair endures the chemical changes caused by a perm.
What happens in a permanent wave? Permanent-wave lotions are designed to penetrate the scales of the cuticle, the outer layer of the hair shaft. The lotions work because they affect the structure of the proteins that make up the hair. The amino acid cysteine, which contains an atom of sulfur, is found in human hair protein. Sulfur atoms in neighboring cysteine molecules within the hair protein form strong, covalent, disulfide (S—S) bonds. This cross-linking between cysteine molecules holds the strands of hair protein in place and affects its shape and strength. When you have a permanent wave, the waving lotion that is applied breaks the disulfide bonds. Then, after the hair is set in a new style, it is treated with a neutralizing lotion, which creates new cross-links between cysteine molecules to hold the hair in the new style. Thus, two chemical reactions occur. In the first, the waving lotion reduces each disulfide bond to two —SH groups. In the second, the neutralizing lotion oxidizes the —SH groups to form new disulfide bonds.
have as the reducing agent ammonium thioglycolic acid. The alkalinity causes the scales of the cuticle to swell and open, allowing the solution to penetrate rapidly. The advantages of the alkaline permanent are that it forms stronger, longer-lasting curls; takes a shorter time (usually 5 to 20 minutes); and occurs at room temperature. The alkaline permanent is used on hair capable of resisting damage. Acid-balanced permanent-wave lotions contain monothioglycolic acid. Normally, acidic lotions penetrate the cuticle only slightly. That’s why the process takes a longer time and requires heat in order for curls to develop. The advantages of the acid-balanced wave are that it forms softer waves, is more easily controlled, and can be used for delicate hair or hair that has been colored. NH O
NH
C
O CH2
CH
S
S
CH2
NH O
C CH NH
Linkage between two cysteine molecules
C
O
C
Exploring Further 1. Investigating Find out more about the protein in hair. Where else is this protein found? 2. Applying Why might hair that has been colored or recently permed absorb lotion more rapidly than untreated hair?
To learn more about the chemistry that goes into hair styling, visit the Chemistry Web site at chemistryca.com
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H H
H H ⫹ H
N
(CH2)6
N
O
s
s
Organic Chemistry
Stretched rubber
s
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s
s
s
s
Unstretched rubber
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H2O
Rubber has a white to brownish-yellow color. Automobile tires are black because the carbon allotrope carbon black is added to strengthen the polymer. 䊳
s
s
s
s
s
s
⫹
Another process that often occurs in combination with addition or condensation reactions is the linking together of many polymer chains. This is called cross-linking, and it gives additional strength to a polymer, Figure 18.22. In 1844, Charles Goodyear discovered that heating the latex from rubber trees with sulfur can cross-link the hydrocarbon chains in the liquid latex. The solid rubber that is formed can be used in tires and rubber balls. The process is called vulcanizing, named for Vulcan, the Roman god of fire and metalworking.
Sulfur links
s
C
Amide link
When a piece of vulcanized, cross-linked rubber such as a rubber band is stretched and then released, the cross-links pull the polymer chains back to their original shape. Without vulcanization, the chains would slide past one another. 䊲 s
N
Rubber
Natural and Synthetic Rubber Latex or natural rubber harvested from rubber trees is soft and tacky when hot. 䊲
s
Hˇ
O
In the second type of polymerization reactions, monomers add on one after another to form chains, as they do in an addition reaction. However, with every new bond that is formed, a small molecule—usually water—is also formed from atoms of the monomers. In such a reaction, each monomer must have two functional groups so it can add on at each end to another unit of the chain. This type of polymerization is called a condensation reaction because a portion of the monomer is not incorporated into the polymer but is split out—usually as water—as the monomers combine. A hydrogen atom from one end of a monomer combines with the —OH group from the other end of another monomer to form water. The condensation reaction used to make one type of nylon is shown in Figure 18.21. Other plastics made by condensation polymerization include Bakelite—which is the hard, ovenproof plastic used for handles of toasters and cooking utensils—and Dacron, which is used as fiber for clothing and carpets, backing on audiotapes and videotapes, and plastic wrap.
Figure 18.22
s
O
H
Condensation Reactions
Condensation Reactions The condensation of two different monomers—1,6diaminohexane and adipic acid—is used to make the most common type of nylon. Nylons are named according to the number of carbon atoms in each monomer unit. There are six carbons in each monomer, so this type of nylon is called nylon 66.
s
C
Adipic acid
Figure 18.21
s
(CH2)4
C
1,6-diaminohexane
s s
O
O
Chemistry and
Recycling Plastics The growing mass of throwaway materials has caused more and more landfills to reach their capacity. In response, recycling has caught on in many parts of the country. People sort their trash into categories: garbage, paper, glass, and plastics. Because garbage and paper are biodegradable and glass can be Code reused, the focus is on plastics. 1 PET Thirty percent of the volume of waste in the United States is composed of plastics. Unfortu2 HDPE nately, recycling of plastics is more complex than for most 3 PVC other materials. Five plastics are commonly found in landfills. 4 LDPE They are polyethylene—both high- and low-density, polyethylene terephthalate, polystyrene, 5 PP polyvinyl chloride, and PS polypropylene. 6
EPS
How are plastics used? Polyethylene is the most widely used plastic. High-density polyethylene (HDPE) is used in rigid containers such as milk and water jugs and in household and motoroil bottles. Low-density polyethylene (LDPE) is usually used for plastic film and bags. Polyethylene terephthalate (PET) is found in rigid containers, particularly carbonated-beverage bottles. Polystyrene (PS) is best known as a foam in the form of plates, cups, and food containers, although in its rigid form, it is used for plastic knives, forks, and spoons. Polyvinyl chloride (PVC) is a tough plastic that is used in plumbing and construction. It is also found as shampoo, oil, and household-product containers. Finally, polypropylene (PP) has a variety of uses, from snack-food packaging to battery cases to disposable-diaper linings. The Society of the Plastics Industry has designated code numbers and acronyms to help people
distinguish among plastics and to make communication about them uniform. The codes are useful in sorting plastics and in deciding what method will be used for recycling. In addition to having a distinct chemical composition, each kind of plastic has distinct physical properties that determine how it can be used. Recycling plastics PET beverage bottles and HDPE milk and water bottles receive the most Polyethylene attention because they are the terephthalate easiest to collect and sort. PET High-density bottles require a drawn-out recypolyethylene cling process because they are made of several materials. Only Polyvinyl the body of the bottle is PET. chloride The base is HDPE, the cap is Low-density another kind of plastic or alupolyethylene minum, and the label has adhePolypropylene sives. The bottles are shredded and ground into chips for proPolystyrene cessing. The adhesives can be Foamed removed with a strong detergent. polystyrene The lighter HDPE is separated from PET in water because one sinks and the other floats. The aluminum is removed electrostatically. What is left are plastic chips that may be sold to manufacturers who can use the chips in making other plastics. However, the FDA prohibits the use of recycled plastics in food containers, making one major market out of bounds for recycled plastics. Material
Analyzing the Issue 1. Debating Debate whether industries should be held responsible for recycling the packaging in which their products are sold. 2. Writing Prepare a letter or article for a newspaper in which you support the use of less packaging for food products. 18.3
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The polymer industry got its start in 1863 when Phelan and Collander, a firm of billiard ball manufacturers, offered $10 000 to anyone who could develop a workable substitute for the ivory then used in making the balls. Ivory was becoming scarce as herds of elephants were killed for their tusks. John and Isaiah Hyatt of New Jersey developed the plastic celluloid in 1870 while trying to make billiard balls, and it became popular for making dental plates, movie film, dice, and stiff collars and cuffs for men’s shirts. However, the Hyatt brothers never got the $10 000 prize because billiard balls made from highly flammable celluloid tended to explode.
When World War II resulted in the cutting off of the Allies’ supply of natural rubber, the polymer industry grew rapidly as chemists searched for rubber substitutes. Some of the most successful substitutes developed were gas- and oil-resistant neoprene, now used to make hoses for gas pumps, and styrene-butadiene rubber (SBR), which is now used along with natural rubber to make most automobile tires. Although synthetic substitutes for rubber have many desirable properties, no one synthetic has all the desirable properties of natural rubber.
Plastics Although the terms plastic and polymer are often used synonymously, not all polymers are plastics. Plastics are polymers that can be molded into different shapes. What physical state has a fixed volume that you can pour into a mold and that will take the shape of its mold? Only liquids will. After a polymer has formed, it must be heated enough to become liquefied if it is to be poured into a mold. After pouring, the plastic will harden if it is allowed to cool, Figure 18.23. Some plastics will soften and harden repeatedly as they are heated and cooled. This property is described as being thermoplastic. Thermoplastic materials are easy to recycle because each time they are heated, they can be poured into different molds to make new products. Polyethylene and polyvinylchloride are examples of this type of polymer. Other plastics harden permanently when molded. Because they are set permanently in the shape they first form, they are called thermosetting polymers. Thermosetting plastics usually are rigid because they have many cross-links. No matter how much they are reheated, they won’t soften enough to be remolded; instead, they get harder when heated because the heat causes more cross-links to form. Bakelite is this type of molecule. Even though thermosetting polymers are more difficult to reuse than thermoplastics, they are durable. Figure 18.24 shows the relative amounts of various plastics produced for packaging in 1987.
Figure 18.23 Molded Plastics Molded plastics are used for making objects with strength and durability qualities that are superior to those of earlier materials. Whereas a wood picnic table eventually will decay, plastic is so difficult to break down that it can create disposal problems as plastic materials pile up in landfills. Many picnic tables are already made of recycled plastic.
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Figure 18.24 Recycling Plastics The relative amounts of various plastics produced for packaging in 1987 are shown here. 87 percent of the plastics sold for packaging are thermoplastic. Polyethylenes and PVC are the most recyclable forms of plastic because they are so easy to remelt and reprocess.
LDPE 32%
HDPE 31%
Other 4%
Connecting Ideas
PVC 5%
Although polymers are large, complex molecules, PET 7% their structures are easier to understand when you realize they are made from chains of simpler building blocks. Living things consist mostly of organic comPP 10% pounds, many of them extremely complex polymer structures. Keep in mind, however, that the chemical reactions through which living things get energy and building materials are exquisitely organized processes whereby a relatively small number of building blocks are used to make the many thousands of biochemicals that cells need. You will study these biomolecules in Chapter 19.
PS 11%
SECTION REVIEW Understanding Concepts 1. Identify the repeating unit that appears in each of the following polymers. a) polystyrene ⬃ CH2CHCH2CHCH2CHCH2CH ⬃
2. Draw the structure of the polymer that will be formed from each of the monomers shown. a) CH2=CHCl b) NH2CH2COOH c) CH2=CHOH 3. Compare and contrast addition and condensation polymerization reactions.
Thinking Critically b) polyvinylchloride ⬃ CH2CHCH2CHCH2CHCH2CH ⬃ Cl
Cl
Cl
4. Making Predictions Can a polymer be made from alkane monomers by addition polymerization? Explain.
Cl
Applying Chemistry
c) Saran Cl
Cl
Cl
Cl
⬃ CH2CCH2CCH2CCH2C ⬃
Cl d) rubber ⬃ CH2
Cl
C
C
CH3
Cl CH2
Cl CH2 ⬃
CH2 C
H
CH3
5. Paint Polymers Paints usually contain three components: a binder that hardens to form a continuous film, a colored pigment, and a volatile solvent that evaporates. In latex paints, one of these components is a polymer. Use what you have learned about polymer properties to decide which of the three most likely is a polymer. Explain.
C H
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CHAPTER 18 ASSESSMENT REVIEWING MAIN IDEAS 18.1 Hydrocarbons ■
■
■
■
Alkanes are saturated hydrocarbons that contain only single bonds between carbon atoms, whereas alkenes and alkynes are unsaturated hydrocarbons. Isomers are compounds that have the same molecular formula but different structures. Because properties depend upon structure, isomers often have different properties. Benzene is one of a group of unusually stable cyclic hydrocarbons. This stability is due to the sharing of six electrons by all the carbon atoms in the molecule. Fossil fuels are our major sources of hydrocarbons. The many components in petroleum are separated by fractional distillation.
18.2 Substituted Hydrocarbons ■
■
■
Special bond organizations, atoms, or groups of atoms that give predictable characteristics to molecules are called functional groups. Atoms and groups of atoms are substituted in carbon rings and chains to form substituted hydrocarbons. Compounds containing a specific functional group share characteristic properties.
18.3 Plastics and Other Polymers ■
Polymers are large molecules made from many smaller units called monomers that
UNDERSTANDING CONCEPTS 1. Draw structures of the following hydrocarbons. a) pentane d) nonane b) propene e) 2-methylpentane c) 1-butyne f) methylpropene
■
■
repeat over and over again. Many synthetic polymers can be made from smaller monomers. Living cells also make many polymers. Addition and condensation are the main reactions by which polymers are made. In addition reactions, all atoms of monomers add end-to-end to form chains. In condensation reactions, a molecule of water is released as each new bond forms between two monomer units. Thermoplastic polymers can be recycled for different purposes because they can be repeatedly softened by heating and hardened by cooling. Thermosetting plastics become hardened permanently.
Vocabulary For each of the following terms, write a sentence that shows your understanding of its meaning. addition reaction alkane alkene alkyne aromatic hydrocarbon condensation reaction cracking cross-linking fractional distillation functional group isomer
monomer polymer reforming saturated hydrocarbon substituted hydrocarbon thermoplastic thermosetting unsaturated hydrocarbon
2. List four natural and four synthetic polymers. 3. What molecule is usually released when monomers combine in a condensation reaction? 4. Name the branched alkanes shown. a)
CH3
CH3
CH2
CH2
CH3CH2CHCH2CHCH2CH3 662
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CHAPTER 18 ASSESSMENT b)
b) ⬃ CH2CHCH2CH ⬃
CH2CH2CH3
OH
CH2 CH3CHCH3 5. Describe how the boiling points of alkanes change as chain length increases. 6. Why is there no compound named 4-methylhexane? What is the correct name for this compound? 7. Distinguish between thermoplastic and thermosetting materials. 8. List two biological functions of esters.
APPLYING CONCEPTS 9. Copy the following structure of penicillin G, an antibiotic. Circle all the functional groups in the molecule. O
H
CH2 C
N
CH C
O Penicillin G
S
CH
CH3 C
N C
H
C
O
CH3
OH 10. Draw the structure of an ether, where R is —CH3 and R′ is —CH2CH3. 11. Predict whether an amine will be more soluble in an acidic or in a basic aqueous solution. Explain. 12. Compare the structures of the following two polymers, and decide which would produce a better fabric to use for making umbrellas. Explain. a) O O ⬃ OCH2CH2OC
C⬃
OH
Biology Connection 13. How many moles of H2 molecules would be required to fully hydrogenate one mole of vitamin A, the structure shown here? CH3
CH3
CH3 CH2OH
CH3 CH3 Everyday Chemistry 14. Compare and contrast the two different kinds of permanent-wave lotions. Chemistry and Society 15. What are the full names and the corresponding codes of the two most commonly recycled plastics?
THINKING CRITICALLY Identifying Patterns 16. The molecular formulas of the noncyclic alkanes follow the pattern CnH2n⫹2, where n is the number of carbon atoms. What pattern is followed by the noncyclic alkenes with one double bond? What pattern is followed by the noncyclic alkynes with one triple bond? Designing an Experiment 17. ChemLab Design an experiment that shows how you could distinguish between two fabric samples if one is polyester and the other is a cotton-polyester blend. Making Predictions 18. MiniLab 1 Predict whether iodine will be decolorized more quickly by melted butter or by melted margarine, assuming that both are at the same temperature. Applying 19. MiniLab 2 What is the name of the ester formed by reacting ethanol and acetic acid?
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Assessment
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CHAPTER 18 ASSESSMENT Making Predictions 20. MiniLab 3 Disposable diapers contain two different polymers. Examine the structures of the polymers shown, and determine which would best function on the inside and which on the outside of a diaper. a) ⬃ CH2 CH2CH2CH2 ⬃ CH2 CH ⬃ b) ⬃ CH2 CH C
O
C
OH
O
OH
SKILL REVIEW 21. Interpreting a Graph Examine the following graph and answer the questions. Boiling Point As a Function of Alkene Branching and Molecule Size
Boiling Point ( C )
70 60 50 40 30 20 10 0 ⫺10 ⫺20 ⫺30 ⫺40 ⫺50 ⫺60 ⫺70 ⫺80 ⫺90 ⫺100 ⫺110
1-hexene 2,3-dimethyl-1-butene 1-pentene 3-methyl-1-butene 2-methyl-1-propene
1-butene
Propene
24. Compare the structures of the following two polymers, and decide which would produce a better fabric to use for making umbrellas. Explain. a) O O ⬃ OCH2CH2OC
C⬃
b) ⬃ CH2CHCH2CH ⬃ OH
OH
WRITING IN CHEMISTRY 25. Do library research to find information about government regulations controlling levels of sulfur in coal-burning emissions. Write a report giving arguments for or against these regulations. If you can think of a better way to reduce sulfur emissions, give your ideas in your report.
PROBLEM SOLVING Ethene 2 3 4 5 6 Number of Carbons in Molecule
a) How does the number of carbon atoms in an alkene affect its boiling point? b) Is there a relationship between the amount of branching and the boiling point of an alkene? If so, what is the relationship?
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22. Hypothesizing The gasoline blend sold in hot climates consists of hydrocarbons of larger molecular mass than the gasoline blend sold in cold climates. Write a report suggesting a reason why refiners might vary the blends in this way. 23. Designing an Experiment Imagine you are a chemist at a textile company and are in charge of a research project aimed at developing a synthetic cotton substitute. What kind of information would you have to collect or questions would you have to ask to solve the problem? Design an experiment for beginning this research.
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26. a) Write the balanced reaction for the synthesis of ethanol from ethene and water. b) If 448 L of ethene gas are reacted with excess water at STP, how many grams of ethanol will be produced? 27. a) Write a balanced equation for the complete combustion of hexane. b) Use the equation you wrote in part a. to determine how many moles of CO2 will be produced from burning 4.25 moles of hexane, assuming 100 percent yield.
Standardized Test Practice
3. Hydrocarbons containing a triple bond are called a) isomers. b) alkynes. c) alkenes. d) alkanes. 4. The most reactive type of hydrocarbon is an a) isomer. b) alkyne. c) alkene. d) alkane. 5. A hydrocarbon with the formula C6H6 will be a(n) a) carbon ring. b) alkane. c) straight chain of carbon atoms. d) bent chain of carbon atoms.
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—
O
CH3CH2CH2 C H
a) ether. b) aldehyde. c) ketone. d) alcohol. 7. What type of compound does this molecule represent? —
—
—
O H H H —
—
H2N— C — C— C— C — H —
2. Isomers are a) compounds with the same chemical formula but different structures. b) compounds with the same structure but different chemical formula. c) compounds with the same number of carbon atoms but different types of bonds. d) compounds with a different number of carbon atoms but the same structure.
6. The compound pictured below is a(n)
— —
1. How many hydrogen atoms will an alkane with 11 carbon atoms have? a) 11 b) 18 c) 22 d) 24
H H H
a) amine b) amide c) ester d) ether 8. A plastic is a a) hardened polymer. b) moldable polymer. c) heated polymer. d) heat resistant polymer.
Test Taking Tip Wear A Watch
If you are taking a timed test, you should make sure that you pace yourself and do not spend too much time on any one question, but you shouldn’t spend time staring at the clock. When each section of the test begins, set your watch for noon. This will make it very easy for you to figure out how many minutes have passed. After all, it is much easier to know that you started at 12:00 (according to your watch) and you’ll be done at 12:30 than it is to figure out that you started at 10:42 and that time will run out at 11:12.
Standardized Test Practice
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