Studies on biologically active acylhydrazones. Part 1. Acid–base equilibria and acid hydrolysis of pyridoxal aroylhydrazones and related compounds

Abstract

A series of pyridoxal aroylhydrazones has been synthesized and their UV, IR and ¹H NMR spectra studied. In neutral methanol these hydrazones exist in the enolimine form, while in aqueous solution (pH ca 7.0) they exist predominantly in the zwitterionic form in which the phenolic proton is transferred to the pyridine nitrogen. Their aqueous acid–base equilibria show three successive steps for which protonation constants have been determined. The different acidity constants are correlated with Hammett substituent constants. The acid hydrolysis reactions of these hydrazones have also been investigated and the attack of a water molecule on the protonated azomethine seems to be the rate-controlling step. The hydrolysis reactions of N-salicylidene- and N-benzylidene-benzoylhydrazines have been studied for comparison.