Isolation of the Stable Boron Trifluoride – Hydrogen Fluoride Complexes of the Methyl-benzenes ; the Onium Salt (or σ-Complex) Structure of the Friedel-Crafts Complexes

Abstract

THE alkylation of aromatic hydrocarbons catalysed by aluminium chloride and similar Friedel-Crafts catalysts is ordinarily accompanied by the formation of deeply coloured oily liquids of high density. The structure of these ‘Friedel-Crafts complexes’ has been investigated by a number of workers1. H. C. Brown and H. W. Pearsal2 have proved the formation of a molecular complex from toluene and aluminium chloride – hydrogen chloride at low temperature. In further work, H. C. Brown and J. D. Brady3 have established the structure of the hydrogen chloride – aluminium chloride complexes of the methylbenzenes as σ-complexes in contrast to the proposed hydrogen chloride complexes which have a π-complex structure (after the representation of Dewar4). The formation of the onium or σ-complexes was proved by Brown by the hydrogen chloride technique by measuring the solubility of hydrogen chloride in the presence of aluminium chloride in the isomeric methylbenzenes.