Tautomers and conjugate base of the nitrenium ion derived from N-acetylbenzidine

Abstract

The mono-N-acetylated ‘nitrenium ion’ derived from the carcinogen benzidine exists at physiological pH as a long-lived mixture of the 4′-amino-4-biphenylyl-N-acetylnitrenium ion 9 and a neutral conjugate base, the mono-acetylated bis-imine 12, with pK_a = 7.5 for the acid–base pair. Also present in very small amounts is the 4′-(acetylamino)-4-biphenylylnitrenium ion 8. This cation is much more reactive than the other two species, and is the source of the major product of the reaction with water. Thus, the N-acetylbenzidine ‘nitrenium ion’ is a very stable system that hides within it a small amount of a highly reactive form.