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

The mono-N-acetyiated 'nitrenium ion' derived from the carcinogen benzidine exists at physiological pH as a long-lived mixture of the 4'-amino-4-biphe nylyl-N-acetyl-nitrenium ion 9 and a neutral conjugate base, the monoacetyl ated bis-imine 12, with pK(a) = 7.5 for the acid-base pair. Also present in very small amounts is the 4'-(acetyl-amino)-4-biphenylylnitrenium ion 8, T his cation is much more reactive than the other two species, and is the sou rce of the major product of the reaction with water. Thus, the N-acetylbenz idine 'nitrenium ion' is a very stable system that hides within it a small amount of a highly reactive form.