A History of the Structural Theory of Benzene - The Aromatic Sextet Rule and Huckel's Rule

Abstract

This paper shows why the aromatic sextet rule rapidly lost significance in the 1930s and why it has been reevaluated since the 1950s. E. C. Crocker first proposed the rule in 1922, and for several years it was considered one of the most probable hypotheses for benzene. In 1931, E. Hückel applied wave mechanics to the benzene problem and proposed a broader rule than the aromatic sextet rule. The author attributes the rapid decline in the rule of six to four factors: disagreement between R. Robinson and Hückel about the induced polarities of substituted benzenes; C. K. Ingold's criticism of the aromatic sextet rule; W. G. Penney's criticism of Hückel's interpretation; and imperfection in Hückel's own interpretation. But a turning point came with the discovery of aromatic seven-membered ring compounds. The fundamental structure of such compounds was first found by W. v. E. Doering in 1954, when he prepared tropylium cation, verifying Hückel's rule for the first time. This led to reevaluation of the aromatic sextet rule and also Bamberger's axiom.