On the nature of the phenylazo group

Three Hammett substituent constants for the p-phenylazo group were determined. The ionization of p-phenylazobenzoic acid in 50% ethanol gives a σ_p value of 0·26. The NBS bromination of p-phenylazotoluene vs toluene in benzene at 80° gives a σ⁺ value of ?0·15. The NBS bromination of 4-phenylazo-3-cyanotoluene vs m-tolunitrile in benzene at 80° give a σ^. value of 0·28. It is concluded that the phenylazo group is a ?I substituent and either a +M or ?M substituent depending on the reaction being studied. This makes the phenylazo group an activator in nucleophilic, electrophilic and free radical reactions.