Equilibration of benzene derivatives: Part I. Bromobenzenes

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Abstract
Relative rates of aluminium bromide catalyzed debromination of bromobenzenes have been determined by competition runs at room temperature. Ortho‐halo‐substituents have an accelerating effect; the influence of m‐ and p‐substituents is also different from that observed in electrophilic substitutions.Isomer equilibria among halo‐bromobenzenes showed low proportions of ortho‐isomers (5‐6%) and meta/para ratios of about 1.7‐2.0; with alkyl‐bromobenzenes, appreciable proportions of ortho‐isomers were found. This difference is attributed to dipole interactions having opposite signs in the two cases. Equilibrium patterns for disubstituted bromobenzenes agreed with isomer distributions computed from mono‐substituted bromobenzenes on the basis of additivity; steric effects may cause deviations, e.g. in the bromo‐ortho‐xylenes.Equilibrium constants for exchange reactions PhZ + PhBr⇄BrPhZ + PhH were found to be 1.45, 1, 0.055, 0.10 and 0.075 for Z = CH3, H, Br, Cl and F, respectively. Combination of these constants with the isomer distributions leads to “partial equilibrium factors” for the exchange of a bromine and a hydrogen atom at a given position in a substituted benzene relative to benzene.