4 Introduction to Aromaticity

Organic Chemistry: Structure and Function

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1 Introduction to Aromaticity Lecture supplement: Thinkbook page 26
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2 Historical Background Street lamps fueled by gas derived from whale and cod oils Unknown liquid condensed in pipes Named benzene •Vapor density experiment reveals MW = 78; formula = C 6 H 6 C:H ratio 1:1 implies high reactivity like acetylene (HC CH) However benzene is fairly inert: Stable at room temperature More resistant to catalytic hydrogenation (H 2 /Pt) than related substances Analyzed by Michael Faraday Conclusion: Benzene is not like related substances. London, 1825
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3 Benzene Structure? Problem remained unsolved for 40 years, until two key ideas formulated: 1857: Fredrich Kekulé suggests carbon can form four bonds 1864: Alexander Crum Brown suggests carbon can form multiple bonds (C=C and C C) Hundreds of C 6 H 6 isomers possible
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4 Benzene Structure? Some possible structures for benzene None of these are correct. Explore these structures with models! Dewar benzene: H H Fulvene: 2,4-Hexadiyne: H 3 C C C C C CH 3 Rearranges to benzene at 25 o C Rearranges to benzene at 90 o C Easy catalytic hydrogenation Easy catalytic hydrogenation X Prismane: X X X
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5 Benzene Structure? Kekulé’s Dream 1866: Solution to the benzene problem comes to Kekulé in a dream. “During my stay in Ghent [Belgium] I resided in elegant bachelor quarters in the main thoroughfare. My study, however, faced a narrow side alley and no daylight penetrated it. For a chemist who spends his day in the laboratory this mattered little.”
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6 Benzene Structure? Kekulé’s Dream “I was sitting writing at my textbook but the work did not progress; my thoughts were elsewhere. I turned my chair to the fire and dozed. Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly in the background. My mental eye, rendered more acute by repeated visions of the kind, could now distinguish larger structures of manifold conformation: long rows, sometimes more closely fitted together all twining and twisting in snakelike motion.”
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