The electronic effect of the phenylazo and t-butylazo groups

Abstract

Hammett σ_p⁺-values for arylazo and t-butylazo groups have been determined by measurements of the kinetics of solvolysis of the appropriately substituted arylpropan-2-yl chlorides. They have been found to be considerably more positive than expected and differ significantly from earlier estimates based on the rates of electrophilic attack on azobenzene. An interpretation of the discrepancy has been advanced based on the differing orientations of the azo linkage with respect to the aromatic ring in the transition state. The introduction of methyl groups into positions ortho to the phenylazo and t-butylazo substituents causes a change in character from –I, –R to –I, +R. This is true not only for the solvolysis reaction but also for benzoic acid ionisation.