Molecular structures of the metastable charge-transfer complexes of benzene (and toluene) with bromine as the pre-reactive intermediates in electrophilic aromatic bromination

Abstract

Successful crystallization and X-ray crystallographic analyses of the highly metastable (1∶1) complexes of bromine with benzene and toluene establish the unique (localized) structure B that differs in notable ways from the long-accepted (delocalized) structure A. Furthermore, we demonstrate the (highly structured) charge-transfer complexes [C₆H₆,Br₂] and [CH₃C₆H₅,Br₂] to be the pre-reactive intermediates that are converted (via an overall Br⁺ transfer) to the Wheland intermediates in electrophilic aromatic bromination. The role of the dative ion pairs [C₆H₆˙⁺ Br₂˙⁻] and [CH₃C₆H₅˙⁺ Br₂˙⁻] in the rate-limiting activation processes is underscored.