Intro to Aromacity - Introduction to Aromaticity Reading...

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68 Introduction to Aromaticity Introduction to Aromaticity Reading from Vollhardt and Schore Sections 15-1, 15-2, and 15-6 Suggested Text Exercises from Vollhardt and Schore Chapter 15: 12, 15, 18, 19, 39, 64 Lecture Supplement Introduction to Aromaticity (Lecture Supplement pages 44–55) Concept Focus Questions 1. In 1825, Michael Faraday isolated a pleasantly aromatic liquid from illuminating gas. Analysis suggested a molecular formula of C 6 H 6 . Draw and name five isomeric structures for this compound, including the correct structure. 2. Treatment of Dewar benzene with sulfuric acid affords Kekulé benzene. What are two important driving forces for this reaction? 3. Explain how each of the following empirical observations suggest that benzene is not an alkene. (a) Benzene does not react with Br 2 . (b) The x-ray crystal structure of benzene shows all the C–C bonds to be equal in length. (c) The heat of hydrogenation of benzene is 36 kcal mol -1 less than three times the heat of hydrogenation of cyclohexene. 4. Discuss the resonance model predictions for the structure of benzene. 5. 1,3-Cyclobutadiene has the same 1:1 carbon to hydrogen ratio as benzene, yet unlike benzene it is very reactive. Explain. 6. 1,3,5,7-Cyclooctatetraene has the same 1:1 carbon to hydrogen ratio as benzene, yet unlike benzene it reacts like a normal alkene. Explain. 7. Define aromaticity. 8. List four structural features necessary for a molecule to be considered aromatic. 9. Classify each structure shown below as aromatic or not aromatic. Explain. O Benzene 1,3,5-Hexatriene Furan Cyclopentadienyl cation
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Introduction to Aromaticity 69 Concept Focus Questions Solutions 1. H 3 C C C C C CH 3 Dewar benzene Prismane Fulvene 2,4-Hexadiyne Kekulé benzene 2. The reaction is driven by the release of a significant level of ring strain (two cyclobutene rings in Dewar benzene) and the formation of an aromatic product. 3. (a) A normal alkene such as ethylene (H 2 C=CH 2 ) reacts readily with Br 2 to form the addition product 1,2-dibromoethane (BrCH 2 CH 2 Br). Because benzene is inert to Br 2 , benzene cannot be a normal alkene. (b) If the actual structure of benzene contained alternating single and double bonds these bonds would also alternate in length. The x-ray crystal structure does not agree with this prediction, so the C–C bonds in benzene cannot be simple alkenes. (c) If benzene were a normal alkene, with three normal C=C bonds, then the heat of hydrogenation for benzene would be three times the heat of hydrogenation for cyclohexene. The fact that the heat of hydrogenation for benzene is 36 kcal mol -1 lower than three times the heat of hydrogenation for cyclohexene indicates benzene is more stable than expected for three cyclohexene-type double bonds. So, benzene cannot be a normal alkene.
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Intro to Aromacity - Introduction to Aromaticity Reading...

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