CHEM&152
Spr ing 2009
ORGANIC STRUCTURES, SHAPES AND ISOMERISM Fill-in
Name __________________________
Date__________
Partner _________________________ Stamp here
Lecturer: ________________________
Discussion In this experiment you will use molecular models to construct 3-dimensional models of organic compounds. You will then represent these models on the paper by writing structural formulas. Structural formulas are more complete than molecular formulas, since they indicate the position of attachment of each atom, and the type of bond formed. The molecular models you construct should give you a better visual understanding of the 3-dimensional nature and spatial relationships that occur in organic molecules, and it is hoped that your understanding of the concept of "isomers" will be enhanced. Your molecular model kit consists of atoms represented as plastic balls that have been drilled to receive connecting bonds. The atoms are colored to represent the different atoms commonly found in organic compounds. All single bonds are represented by placing a shor t-length (hard) gray plastic stick between the 2 atoms. Unsaturated (double and triple bonds) linkages (or highly strained linkages) require the use of long-length (flexible) gray plastic sticks. The following color identification scheme is used for the models you will be working with: C = black; H = white; Cl = green; N = blue; O = red The IUPAC naming system The IUPAC naming system was developed to provide a systematic method of naming organic compounds. It is designed with the following fundamental principle: each differ ent compound should have a differ ent name. This discussion will provide a brief introduction to the IUPAC system. 1. Each IUPAC name consists of several parts: 3-D relationship, numbered substituents, the number of carbons in the longest chain and an ending (suffix) indicating functional groups (base name). 2. Count the number of carbons in the longest, continuous chain; this may or may not be equal to the total number of carbons in the structure. This gives you the base name, which has a root name based on the number of carbons (See Table 1) and one of the following endings: -ane: all C-C single bonds
-ene: one C-C double bond
-yne: one C-C triple bond.
3. Number the carbon chain. You can number from left to right, or right to left; however the substituents (groups coming off of the chain) should have the lowest possible numbers. 4. Name the substituents as alkyl groups, with a number in front giving the location of this group. An alkyl group name typically consists of the prefix, based on the number of C’s in the group, and the suffix –yl. Halogens get a slightly different name: chlorine becomes ‘chloro’, bromine becomes ‘bromo’. Substituent names are listed in alphabetical order, not numerical order. This part of the name comes before the root name of the longest chain. 5. When two or more substituents are identical, they are listed together. Indicate this by the use of the prefixes di-, tri-, tetra-, etc. Give each substituent its own number. 6. Use hyphens to separate numbers and letters, commas to separate numbers. Chemistry&152
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Some examples: note: there are several different ways to draw structures. You can have expanded structures, and condensed structures. Structures are generally drawn in a “ linear” fashion for ease, but remember that a C-C single bond does not have an angle of 180o but instead an angle of 109.5o. For that reason, it is easier to draw the structures in a line, but note the two examples below for the same structure. CH 3
H
CH3
Cl
C
H
H3 C
C
C
H
H
CH2 CH 3
H
C
C H
H
C
C H
H
H
Cl
H
H
A more condensed “linear” structure and an expanded structure with correct bond angles. Both represent the molecule: 2-methyl-3-chloro pentane (prefix – substituents) root (5 carbon chain) suffix (ane for all C-C single bonds)
H3 C
CH3
CH3
C
C
H
CH3
CH2 CH 2CH 3
2, 3, 3-trimethylhexane Table 1: The root names for 1 through 10 for hydrocarbon groups are listed below, and should be memorized. car bon #
r oot
car bon #
r oot
1 2 3 4 5
methethpropbutpent-
6 7 8 9 10
hexheptoctnondec-
The Exper iment A. The Tetr ahedr al Car bon Atom 1. Examine one of the carbon atoms. How many holes are found in the model carbon atom? _______ 2. Insert a shor t gray stick into each hole of the carbon atom. What do these sticks represent? ___________ 3. How many covalent bonds are necessary for each carbon atom in an organic molecule? _______
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4. What is the ideal bond angle represented by any pair of sticks in a carbon with only single bonds; note that all pairs of sticks have identical bond angles. _______ 5. What causes these bonds to be symmetrically arranged about the carbon atom? ___________________________________________________________________________
B. Alkanes and Alkyl Gr oups. Alkanes are hydrocarbon compounds containing only single bonds. These are considered satur ated. They are named according to the longest continuous chain of carbon atoms. 1. Construct a model of a methane molecule by adding a hydrogen (white ball) to each of the 4 bonds in your carbon model. Draw the structural (2-dimensional) formula represented by this model.
This 1-carbon group is called methane, and is the simplest alkane. It, and all alkanes, follow the generic formula of CnH(2n + 2), where n represents the number of carbon atoms in the longest continuous chain. 2. Remove one of the hydrogens (the white ball) -but not the bond/stick - from your methane model. This is the simplest example of an alkyl group. Draw the structural formula for this group. Alkyl groups are carbon containing chains or groups that are connected to a longer carbon chain.
This 1-carbon group is called a methyl group, and has the formula -CH 3. Alkyl groups of larger carbon number have the general formula of C n H (2n + 1). This is the simplest of the alkyl groups. Alkyl groups are named by using the root indicating the number of carbons in the group and adding -yl to the root. a. What is the name for a 5-carbon straight chain alkyl group? _________________ b. What would be the formula for an ethyl group (2 carbon atoms)? ________ c. Draw the structural formula for the ethyl group
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d. Construct a model of this ethyl group. Rotate the molecule by grasping one methyl group in each hand. Is free rotation about a single bond possible? Yes
No
e. Place a white ball (hydrogen) on the structure so that all bonds are connected to atoms (2 carbons, and 6 hydrogens total). Rotate the molecule until the hydrogens are in a "staggered" arrangement. Rotate the molecule again until the hydrogens are directly over each other (eclipsed). This representation of ethane is called a Newman Projection.
“staggered”
“eclipsed”
Reference: http://www.chem.unl.edu/cak/images/bk6.ht41.gif
f. Which conformation (form) would you predict would be favored? (Remember that the sticks represent negatively charged electron pairs.) ________________ g. Are all six hydrogens of the ethane molecule chemically equivalent? Yes
No
3. Remove a hydrogen from ethane, and unite the ethyl group with a methyl group. Draw the structural formula for this new hydrocarbon.
a. What is the molecular formula for this new compound? b. What is the name of this compound?
______________ ____________________
c. Examine the arrangements of hydrogen atoms in this new model. Remove a hydrogen from the end carbon. Examine the structure. Replace it. Remove a hydrogen from the other end carbon. Examine the structure. How are those two structures related - are they all chemically equivalent (meaning, are they the same structure or different)? Yes they are equivalent
No, they are different structures
d. Now remove a hydrogen from the end carbon, examine the structure. Replace it. Remove a hydrogen from the middle carbon, examine the structure. Replace it. How are those two structures related – are they chemically “equivalent” (aka the same)? Yes they are equivalent #1 Organic Structures
No, they are different structures
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4. Straight chain alkanes all have the formula CnH(2n+2). They are all named by finding the longest continuous carbon chain and following the nomenclature system of : prefix-root-ane. Prefixes will be used if there are substituents (anything other than hydrogen) bonded to the main chain; root represents the number of carbons in the longest chain and -ane indicates that the structure is comprised only of carbon-carbon single bonds. Using this information, draw the following structures: a. Butane: b. Pentane: c. Octane: And name the following structures: a.
CH3CH2CH2CH2CH2CH3 ________________________________________
b. CH3CH2CH2CH2CH2CH2CH3 ______________________________________ Now try a few with substituents (side chain groups) a.
2-methyloctane
b. 3-ethylnonane
5. Atoms other than carbon and hydrogen can be bonded to carbon atoms. From a 2-dimensional drawing of dichloromethane (CH2Cl2) it appears that two different dichloromethanes exist. Cl
Cl H—C—H
and
Cl—C—H
Cl
H
Construct a model of dichloromethane by replacing 2 of the white balls on methane with green balls. What does your 3-dimensional model tell you about the "equivalency" of the 2dimensional models of dichloromethane shown above? (circle your answer below) They are equivalent/the same structure
They are different structures
Briefly explain your circled answer: #1 Organic Structures
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C. Isomer ism Isomers (iso = same; mer = part) are compounds that have the same molecular formulas but differ in either position/connectivity (structural isomer) or 3-dimensional shape (geometric isomer) of attachment. Molecules are not isomers if they can be turned into one another without breaking any bonds. 1. Join 4 carbon atoms together by shor t single bonds then fill out the structure by adding hydrogen atoms. a. Is it possible to do this in more than one way?
________
b. Draw 2 differ ent str uctur al formulas for a compound containing four carbons and 10 hydrogens.
c. How do the molecular formulas for these two compounds compare?_________________ d. What ter m is used to describe compounds with the same molecular formula but different structural formulas (circle the correct term)? Structural isomer
Geometric isomer
2. There are thr ee possible structural isomers of pentane (C 5H 12). Use your model kit to determine these structures and draw structural formulas for these isomers.
3. There are many other examples of isomerism in organic chemistry. Construct models and draw the four structural isomers of dichloropropane (C 3H 6Cl2).
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D. Cycloalkanes - Hydr ocar bon compounds ar r anged in cir cles. 1. Construct a model of cyclopropane: CH2 CH2
CH2
a. Can this be easily done using the short plastic sticks?
_________
b. What is the bond angle that results when you force the 3 carbon atoms to bond in this way? (Think triangles!) _________ 2. Construct a model of cyclobutane: CH2
CH2
CH2
CH2
a. What is the bond angle between carbons in this molecule?
_________
b. What should the ideal bond angle be for C-C single bonds?
_________
c. Do you think that cyclopropane is stable? Do you think that cyclobutane is stable? (hint: a stable structure is one in which the atoms are not strained in their bonding) __________________________________________________________________ 3. Construct a model for cyclohexane: CH2 CH2
CH2
CH2
CH2 CH2
a. Note that by puckering and buckling, the cyclic structure can have two different structures (conformations), a boat and chair form. Try this with your structure.
boat
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chair
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b. Do the bonds in this structure appear strained in any way? Yes
No
c. What is the approximate bond angle between carbon atoms in this molecule (remember these are all singly bonded carbons!)? _________ d. Would you expect cyclohexane to be more or less stable than cyclopropane? Explain using the concept of bond str ain
E. Alkenes and Cis-Tr ans (Geometr ic) Isomer ism. Alkenes are hydrocarbons containing one or more double bonds. They are unsatur ated. They are named according to the longest continuous chain of carbon atoms, adding –ene to the end, and using a number to indicate the position of the double bonds, if necessary. The simplest alkene is ethene, CH2=CH2, also known by the common name, ethylene. 1. Make a model of butane: CH3 - CH2 - CH2 - CH3. a. Rotate the single carbon-carbon bonds such that the end methyl (CH 3) groups are on opposite sides of the center carbons. b. Rotate the single carbon-carbon bonds such that the end methyl groups are on the same side of the center carbons. c. Would the structures you have produced in parts a and b be considered isomers of one another? Explain the basis for your answer.
2. Make a model of cis-2-butene (both methyl groups are on the same side of the double bond). The structural formula for cis-2-butene is shown below: CH3
CH3 C=C
H
H
cis-2-butene
a. Make another model of trans-2-butene (the end methyl groups are on the opposite sides of the carbon-carbon double bond).
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b. Can your cis-2-butene model be converted into the trans-2-butene model without breaking bonds? Meaning, can you rotate the carbon atoms around the double bond? Yes
No
c. Draw the structural formula for trans-2-butene.
d. The cis- and trans-2-butenes are called geometric isomers. Why are they given this name?
e. Define the term structural isomer and explain how it differs from a geometric isomer.
3. Construct a model of the benzene molecule from the structural formula. Benzene is probably the most recognizable example of an ar omatic hydrocarbon (it has alternating double and single bonds, in a ring), and a specialized alkene. Two different representations of benzene are shown below- the Lewis structure and a shorthand version. H
H
H C C
C
C
C C
H
H
H
a. What is the hybridization and bond angles on each of the carbons in benzene? Hybridization = _____________
Bond angle = _______________
b. Draw the structural formula for 1,3-dimethylbenzene.
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4. Indicate whether each of the following compound pairs are geometric isomers, structural isomers, or not isomers at all. Write the names for the compounds under each formula. a. ________________ CH3 - CH = CH - CH3
and
______________________________
CH3 - CH2 - CH = CH2
and
_____________________________
b. _________________ Br
Br
Br C
and
C
H
H
H
______________________________
C
C
H
and
Br
____________________________
c. _________________ CH3 CH3
CH
CH3
CH3
______________________________
and
CH 2
CH 2
CH3
_____________________________
d. _________________ Cl CH3
CH
CH
2
and
CH3
Cl
Cl
CH
CH
2
Cl
_______________________________ #1 Organic Structures
and
____________________________
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F. Alkynes. Alkynes are hydrocarbons containing one or more triple bonds. They are unsatur ated. They are named according to the longest continuous chain of carbon atoms using a number to indicate the position of the triple bonds, if necessary. The simplest alkyne is ethyne, CH≡CH, also known by the common name, acetylene. 1. Construct a model of 1-butyne and draw its structural formula.
2. Construct a model of 2-butyne and draw its structural formula.
3. What is the only difference between 1-butyne and 2-butyne?
4. What chemical term describes molecules like 1-butyne and 2-butyne - they are called? _________________________________ G. Other Functional Gr oups. The behavior of organic molecules is governed by the presence of functional groups, combinations of atoms that act as chemically reactive sites. Compounds with the same functional group will have many similar properties and undergo similar chemical reactions. Double and triple bonds are also functional groups. Build the following molecules. 1. Alcohols-alcohols have the general formula R-OH, where R= any alkyl group and the OH is called a hydroxy group. (Note: since the OH is bonded covalently to a carbon, it does not come off as a hydroxide ion! In other words, this is not an ionic compound.) Again, the longest carbon chain will be the chain that contains the –OH functional group and the OH will always get the lowest number in the molecule. a. Construct a model then draw the structural formula for 1-propanol, C3H7OH. (The common name is propanol, or propyl alcohol)
b. Construct a model of 3-propanol. Explain why this name is not correct.
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c. You most likely left something off your Lewis structure above. What is missing from the oxygen that would complete its octet? (Think back to Lewis Structures) ___________________ d. Construct a model, then draw the structural formula for 2-propanol, C3H7OH. (The common name is isopropanol, or isopropyl alcohol).
Name the following alcohol, using the examples above: OH CH3CH2CH2CHCH3 _____________________________________
2. Ether s have the general formula R-O-R', where R,R' = alkyl groups. a. Construct a model then draw the structural formula for dimethyl ether , C2H6O. Ethanol has the same molecular formula. Draw the structure for ethanol
Dimethyl ether
Ethanol
b. What chemical term describes structures like ethanol and dimethyl ether? These structures are called _____________________________________
O C 3. Aldehydes and ketones have the general formula R groups. For aldehydes, at least one R must be hydrogen.
R' , where R and R' = alkyl
a. Construct a model then draw the structural formula for the simplest aldehyde, methanal, CH2O. (The common name is formaldehyde). Remember carbon must always have 4 bonds. #1 Organic Structures
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b. Construct a model then draw a structural formula for propanal, C3H6O.
c. Construct a model then draw the structural formula for the very common ketone, 2propanone, C3H6O. (The common name is acetone)
d. What chemical term describes structures like propanal and propanone? These structures are called _________________________________ O
4. Car boxylic acids (COOH) have the general formula R C O-H where R = an alkyl group. When naming a carboxylic acid, the carbon attached to the oxygens is the first carbon in the chain, and the longest chain beginning with that carbon sets the prefix for the name. The –e is dropped from the alkane chain name, and –oic acid is added. a. Construct a model, then draw the structural formula for ethanoic acid, C2H4O2 (The common name is acetic acid).
b. Now do the same for 3-chloropropanoic acid.
c. Acids can be derived from benzene, where the carboxylic acid group (COOH) is attached to the benzene ring. Construct a model for benzoic acid (C7H6O2 or C6H5COOH) and draw it. Then construct a model and draw the structural formula for 4-methylbenzoic acid (C8H8O2) Benzoic Acid
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4-methylbenzoic acid
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O C O R' 5. Ester s have the general formula R where R, R' = alkyl groups. Esters are named by starting with the alcohol, named as an alkyl group, and then ending with the carboxylic acid, removing the –ic acid and replacing it with the suffix -ate.
a. The following steps will help you construct a model of ethyl butanoate. b. First: ethyl represents which part of the molecule – the alcohol, or the acid? Make this molecule – draw it below:
c. Second: the butanoate represents which part of the molecule – the alcohol, or the acid? Make this molecule – draw it below:
d. Finally, bring the two molecules together – OH end to OH end. Remove the OH from the acid (remove the bond –OH also) and one hydrogen (white ball only) from the alcohol (leave the bond still attached to the alcohol), connect/bond the remaining molecules together. By the loss of water, you have created an ester!
e. Construct a model then draw the structural formula for ethyl ethanoate, C4H8O2. (The common name is ethyl acetate, the smelly stuff in nail polish remover!)
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6. Amines have the general formula of R3N, where R is any alkyl group or hydrogen. They are named by listing the names of the three alkyl groups, followed by the –amine suffix. a. Draw the structure for ammonia, NH3, the simplest amine.
b. Again, you probably took a few shortcuts when drawing your NH3 molecule. What did you leave off from your Lewis structure that would complete the octet on the N? _________________ c. Construct a model then draw the structural formula for triethylamine, N(CH2CH3)3. This compound has a “fishy” smell. (note the N is attached to 3 –CH2CH3 groups!)
O R
C
N
7. Amides have the general formula . They are formed by combining a carboxylic acid and an amine and removing a water molecule. a. Construct a model then draw the structural formula for the amide formed when ethanoic acid reacts with ammonia (hint: follow the same procedure for the loss of water that you did when making the ester on the previous page).
b. Construct a model then draw the structural formula for benzamide, C7H7NO (or C6H5CONH2).
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