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ORGANIC CHEMISTRY
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Isomerism
Types of Organic Compounds
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• Isomers have identical composition but different structures • Two forms of isomerism
• Vast majority of over 20 million known compounds are based on C: organic compounds. • Generally contain C and H + other elements • Great variety of compounds
– Constitutional (or structural) – Stereoisomerism
• Constitutional – Same empirical formula but different atomto-atom connections
• Stereoisomerism – Same atom-to-atom connections but different arrangement in space.
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Structural Isomers
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Stereoisomers: Stereoisomers: Optical
Stereoisomers: Stereoisomers: Geometric Geometric isomers can occur when there is a C=C double bond.
• Optical isomers are molecules with non-superimposable non-superimposable mirror images. Such molecules are called CHIRAL Pairs of chiral molecules are enantiomers. enantiomers. • Chiral molecules in solution can rotate the plane of plane polarized light. • •
Cis-2butene Cis-2-butene
Trans-2-butene Trans-2-butene
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Chiral Compounds and Polarized Light
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Stereoisomers Isomers
Stereoisomers Isomers
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Chirality generally occurs when a C atom has 4 different groups attached. Lactic acid isomers are nonsuperimposable
Lactic acid
Chirality: Chirality: Handedness in Nature
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Chirality: Chirality: Handedness in Nature
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Stereoisomers in Nature
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Right- and lefthanded seashells These molecules are non-superimposable mirror images.
These amino acids are nonsuperimposable mirror images.
The DNA here is right-handed
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Hydrocarbons
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Hydrocarbons • Alkanes have the general formula CnH2n+2 • CH4 = methane • C2H6 = ethane • C3H8 = propane • C4H10 = butane • C5H12 = pentane
• Compounds of C and H • Subgroups: – Alkanes: Alkanes: C-C single bonds – Alkenes: Alkenes: C=C double bonds – Alkynes: Alkynes: carbon-carbon triple bonds – Aromatic: based on benzene
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Methane Hydrate, CH4(H2O)x
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Methane Hydrate, CH4(H2O)x
Methane Hydrate, CH4(H2O)x Gas hydrates have been known for many years, and combustion of a sample of methane hydrate is seen on the front cover. Recently, however, vast deposits of methane hydrate were discovered deep within sediments on the floor of the world’s oceans. How these deposits were formed is a mystery. But what is important is their size. It is estimated that the global methane hydrate deposits contain approximately 1013 tons of carbon, or about twice the combined amount in all reserves of coal, oil, and conventional natural gas. Now if scientists and engineers could only solve the problem of extracting the methane conveniently and safely!
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CH3CH2 CH2CH2 CH 3 Pentane
CH3
Hydrocarbons 19 & Structural Isomerism
CH3CHCH2CH3
Isomers of C 5H12?
2-Methylbutane
CH3 H3CCCH3 CH3 2,2-Dimethylpropane
Note names of isomers
C5H12 has 3 structural isomers. C6H14 has 5 C7H14 has 9
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Hydrocarbons: Alkanes
3 4 CH2CH3 1 C
H
1 C H
1-butene
3 CH3
H
2 C H
1 H3C
2 C
4 CH3 2 C
CH3
2-methylpropene (isobutene)
H
H
cis-2-butene
1 H3 C
H
H
H 2
3
H H
Equatorial H atom
H 5
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6 1
H
1
H
6
5
H
HH
3
H H
H
2
H H
H
H
H 4
H
H
H H
Boat form
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H H
4
H 3
H
H 1
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H HH
H
H
2
H
Chair form
Reactions of Alkenes: Alkenes: ADDITION REACTIONS
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H2O add to the double bond
4 CH3 2 C
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• Alkenes are unsaturated — more bonds can form to the C atoms • Molecules such as Br 2, H2, HCl, HCl, HBr, HBr, and
H
3 C
Axial H atom
HH
Chair form
• How many isomers are possible for a compound with the formula C 4H8? H
All compounds are flexible. Cyclohexane, Cyclohexane, C6H12, has interconverting “chair” and “boat” forms.
Alkanes are colorless gases, liquids, and solids Generally unreactive (but undergo combustion) Not polar (or low polarity) and so are not soluble in water.
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Alkenes: Alkenes: Compounds with C=C Double Bonds
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Hydrocarbons: Cycloalkanes
3 C
H C C + Br2 H H H
H
trans-2-butene
Alkenes— Alkenes— Many Occur Naturally
Br Br H C C H H H 1,2-dibromoethane
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An Addition Reaction
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Fat placed in Br2 vapor
An Addition Reaction
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• Fats can be “hydrogenated” with H2.
Fat placed in Br2 vapor
• The fat in bacon is partially unsaturated. The fat adds Br 2 to the C=C bonds. • Fats can be “hydrogenated” with H2.
Alkynes • Alkynes have carbon-carbon triple bonds. • C2H2: common name = acetylene systematic name = ethyne Preparation: CaC22(s) + H22O(liq liq)) --> O( O(liq) C22H22 (g) + Ca(OH) 22(s) Hffoo(C22H22, g) = +226.7 kJ/mol kJ/mol Hrxn rxn for C22H22 + O22 = –1300 kJ/mol kJ/mol
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An Addition Reaction
Peanut butter has partially hydrogenated vegetable oil.
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Resonance in Benzene
Aromatic Compounds • Benzene, C 6H6, in the top 25 chemicals produced in the U.S. • Starting point for hundreds of other compounds.
• C6H6 has two resonance structures with alternating double bonds. electrons are delocalized over the ring.
• The H
C
HC
H C
H
H
CH
H
C C H
C
H C
C C H
C
H
CH
Resonance structures of benzene, C
6 H6
H
C
HC
H C C C H
H
CH
Abbreviated representation of resonance structures
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Resonance in Benzene
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Other Aromatic Hydrocarbons
Benzene Derivatives
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• CC bond order is _______________ • C–C single bond = 154 pm C=C bond = 134 pm • CC bonds in benzene = 139 pm electrons delocalized
Toluene
Naming Benzene Derivatives Cl
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1 2
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3
5
Aniline
Phenol
C6H5NH2
TNT trinitrotoluene
C6H5OH
C6H4CH3(NO2)3
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Reactions of Aromatics • Substitutions — not additions — are typical.
Ortho to Cl
CH3
Meta to Cl
+ CH3Cl
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Naphthalene
AlCl3
+ HCl
Para to Cl 1,4-dimethylbenzene Common name: Para-xylene
AlCl3 is a catalyst. Catalysts typically used in aromatic substitutions.
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