Add time:08/10/2019         Source:sciencedirect.com

Three Hammett substituent constants for the p-phenylazo group were determined. The ionization of p-phenylazobenzoic acid in 50% ethanol gives a σp 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.

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