chapter 11 - Chapter 11 Arenes and Aromaticity Chem 211 B R...

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Unformatted text preview: Chapter 11 Arenes and Aromaticity Chem 211 B. R. Kaafarani 1 Examples of Aromatic Hydrocarbons H CH CH 3 H H H H H H H H H H H H Benzene Toluene H H H H Chem 211 B. R. Kaafarani 2 Naphthalene H H 11.2. The Structure of Benzene  Kekulé (1866) proposed a cyclic structure for C 6 H 6 with alternating single and double bonds. ater ekulé vised is roposal y uggesting pid  Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. owever is roposal uggested omers f e ind hown  However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. X X H X X H H H H H Chem 211 B. R. Kaafarani 3 H H Structure of Benzene  Structural studies of benzene DO NOT support the Kekulé formulation. Instead of alternating single and double bonds, all of the C—C bonds are the same Benzene has the shape of a regular hexagon Chem 211 B. R. Kaafarani 4 All C—C bond distances = 140 pm 40 pm 40 pm 40 pm 40 pm 46 pm 46 pm 140 pm 140 pm 140 pm 140 pm 146 pm 146 pm 140 pm 140 pm 140 pm 140 pm 140 pm 140 pm 140 pm 140 pm 134 pm 134 pm  140 pm is the average between the C—C single bond Chem 211 B. R. Kaafarani 5 distance and the double bond distance in 1,3-butadiene. Resonance Picture of Bonding in Benzene  Instead of Kekulé's suggestion of a rapid equilibrium between two structures. xpress e tructure f enzene s sonance  Express the structure of benzene as a resonance hybrid of the two Lewis structures. Electrons are not calized lternating ingle nd ouble onds, ut re localized in alternating single and double bonds, but are delocalized over all six ring carbons. H H H H H H Chem 211 B. R. Kaafarani 6 H H H H H H Resonance Formulation of Benzene  Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures). Chem 211 B. R. Kaafarani 7 11.3. The Stability of Benzene  Benzene is the best and most familiar example of a substance that possesses "special stability" or "aromaticity”.  Aromaticity is a level of stability that is substantially greater for a molecule than would be expected on the basis of any of the Lewis structures written for it. Chem 211 B. R. Kaafarani 8 Thermochemical Measures of Stability  Heat of hydrogenation: compare experimental value ith "expected" value for hypothetical “ yclohexatriene” with expected value for hypothetical cyclohexatriene . + 3 H 3H 2 Pt Pt  H°= = – 208 kJ 208 kJ Chem 211 B. R. Kaafarani 9  Observed heat of hydrogenation is 152 kJ/mol less than "expected“.  Benzene is 152 kJ/mol more stable than expected.  152 kJ/mol is the resonance energy of benzene. Figure 11.2 (p. 433) Chem 211 B. R. Kaafarani 10 Cyclic conjugation versus noncyclic conjugation 3H 3H 2 Pt Pt Heat of hydrogenation = 208 kJ/mol Heat of hydrogenation = 208 kJ/mol 3H 3H 2 eat of hydrogenation = 337 kJ/mol eat of hydrogenation = 337 kJ/mol Pt Pt Chem 211 B. R. Kaafarani 11 Heat of hydrogenation = 337 kJ/mol Heat of hydrogenation = 337 kJ/mol Resonance Energy of Benzene...
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chapter 11 - Chapter 11 Arenes and Aromaticity Chem 211 B R...

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