Extraordinary large stabilization of the phenyl cation by hyperconjugation with strained bonds. Experimental confirmation of a theoretical prediction

Abstract

Appearance energy measurements for the loss of Br˙ from ionized 2-bromocyclopropabenzene (5c), in conjunction with thermochemical data, suggest that the 2-cyclopropaphenyl cation (2) is stabilized, relative to the phenyl cation (1a), by at least 27.6 kcal/mol, in very good agreement with a previous prediction based on ab initio MO calculations ( 23 kcal/mol at 3-21G*); the heat of formation of (2) is estimated to be 311 kcal/mol (1 cal = 4.184 J).