C&EN feature DR. RONALD BRESLOW,
Department of Chemistry, Columbia University, New York, N.Y.
Aromatic Character One hundred years after Kekulé, chemists are still trying to understand aromatic character. Modern techniques and the increasing application of quantum mechanics to organic chemistry have led to the preparation of many new aromatic systems and to a deeper understanding of conjugated systems generally
Benzene was discovered by Michael Faraday in 1825. Its formula, C 6 H 6 , indicated a high degree of unsaturation. However, in its chemical properties benzene is quite unreactive compared with simple olefins. The puzzle of its structure was to occupy the attention of many chemists; only with the application of quantum mechanics to organic structure theory, in the 1930's, was the problem brought to its current state of understanding. However, a major advance was Kekulé's deduction, in 1865, that benzene has a six-membered cyclic structure. Some years later, he noted that "oscillation" between two different bond arrangements is required to account for the number of known isomers in substituted benzenes—for example, only one o-dibromobenzene. H
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The problem of bonding in benzene was not really solved by this suggestion. Kekulé's oscillation accounted for the apparent hexagonal symmetry of benzene, but it did not seem to explain why benzene is so remarkably different from ordinary olefins. For this reason, other authors retained the hexagonal carbon skeleton, but proposed special bonding schemes to make it clear that benzene does not have ordinary double bonds. 90
C&EN
JUNE
2 8,
1965
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