The electronic effect of the phenylazoxy group

Abstract

Hammett σ_m and σ_p values have been determined for the phenyl-ONN-azoxy and phenyl-NNO-azoxy groups by measuring the pK_a values of the phenylazoxybenzoic acids in 50% ethanol. The σ_p⁺ value for phenyl-ONN-azoxy has been evaluated both from the rate of solvolysis of 4-phenyl-ONN-azoxyphenyldimethylcarbinyl chloride in aqueous acetone, and from the rate of bromination by acidified hypobromous acid. The two values obtained differ markedly, and the variations are interpreted in terms of differences in the geometry of the transition states for the two reactions.