Kinetic Instability of Nonbenzenoid Aromatic Hydrocarbons

Abstract

Abstract Aromatic hydrocarbons have thermodynamically stable cyclic conjugated systems. There is a tacitly accepted view that not only aromatic stabilization but also kinetic stability arise from the same cyclic conjugation of π electrons. In order for this view to be examined criticaly, two reactivity indices, i.e., localization energy and superdelocalizability, were adopted for a variety of cyclic conjugated hydrocarbons, and were compared with those defined for their olefinic reference structures. Interestingly, it was found that chemical reactivity of some constituent carbons might often overcome aromatic stabilization of an entire conjugated system. Not a few nonbenzenoid aromatic hydrocarbons were predicted to have reactive carbons which are olefinic in reactivity. These hydrocarbons constitute unstable aromatic compounds. Thus, it has been clarified that thermodynamic and kinetic stabilities do not always concur in the case of nonbenzenoid hydrocarbons.