Dynamics of theory change in chemistry: Part 2. Benzene and molecular orbitals, 1945–1980

Abstract

In my previous article on the benzene problem, I described how Pauling's valence bond (resonance) theory, sometimes regarded as a modernized version of Kekulé's oscillation hypothesis, came to be accepted by chemists by the end of World War II. But the alternative molecular orbital theory, proposed by Mulliken, had already been developed and was regarded as quantitatively superior by many quantum chemists, though it was not as easy to visualize and did not seem to harmonize as well with traditional chemical concepts. During the 1950s and 1960s, thanks to the efforts of Charles Coulson and many other theorists, the molecular orbital approach not only dominated theoretical discussions but also started to be accepted by the chemical community as a whole and became the preferred description for benzene. Possible reasons were: its greater calculational convenience when applied to large molecules; better expository methods directed toward chemists; the spectacular success of the Woodward–Hoffmann rules for pericyclic reactions and Fukui's frontier orbital theory; and the development of a general theory of aromaticity, which predicted properties of similar molecules such as cyclobutadiene (C 4H 4). The relative importance of these reasons is explored through a mail survey of chemists.