The halogenation of aromatics: Part VII. Isomerizations, disproportionations and transbrominations involving bromobenzenes in the presence of aluminium bromide

Abstract

The dibromobenzenes isomerize under the influence of aluminium bromide at room temperature to form an equilibrium mixture of dibromo compounds (60 % meta, 35 % para, 5 % orrho), bromobenzene and tribromobenzenes.p-Dibromobenzene first forms ortho-isomer, meta-isomer being formed at a later stage. Small amounts of water have an accelerating effect (formation of HBr); carbon disulphide or naphthalene are inhibitors, probably as a result of complex formation involving aluminium bromide.The intermolecular nature of these reactions is demonstrated by the fact that in the presence of an excess of acceptors such as chlorobenzene or toluene, transbromination (ArBr + ArZ -+ ArH + ArZBr) is the main reaction observed.This phenomenon is used for the determination of relative rates of debromination of bromobenzenes (PhZBr) in competition runs: (Z = H unity), p-C1 0.6, p-Br 0.4 (x2), m-Br 0.03 (x2), 3,5-diBr<0.01. Equilibrium constants are presented for several processes of the type ArBrZ + ArY f ArZ + ArYBr.Results are discussed in terms of an intermediate u-complex ArBrHf , which niay lose either a proton or a bromine cation. Equilibria may be correlated with data on bond strengths (Dc-nr) in substituted bromobenzenes. IntroductionIn part I of this series la attention was called to the aluminium chloride catalyzed isomerization of dichlorobenzenes in connection with the peculiar isomer patterns obtained in gas phase chlorinations of chlorobenzene.The isomerization of dibromobenzenes was studied briefly many years ago by several authors 2 and more recently by Wibaut, Sixma and Lips 3 .