ortho-Group participation in azocarbonium ion and 1,3-dipolar ion formation

Abstract

The rates of solvolysis of N-(p-nitrophenyl)-o- and -p-nitrobenzhydrazonyl bromides (hydrazidic bromides) in 70% dioxan have been studied as a function of pH. At pH > 3, the rate-determining step is 1,3-dipolar ion formation. The products of solvolysis are 3-(p-nitrophenylazo)anthranil 1-oxide and N-(p-nitrophenyl)-p-nitrobenzhydrazide from the ortho- and para-isomers. The isomers, however, solvolyse at approximately the same rate at constant pH. Rate-determining azocarbonium ion formation was also studied by use of similar substrates in 25% dioxan (50°C) at pH 1·0. All ortho-groups in the arylidene ring accelerated the ionization of the hydrazonyl bromide (k_ortho/k_para for NO₂, F, Br, and Cl being 65, 1·9, 7·4, and 3·1 respectively). This rate enhancement is rationalized in terms of stabilization of the incipient carbonium ion by the field effect of the substituent.