A series of pyridoxal aroylhydrazones has been synthesized and their UV, IF? and 'H 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 t o be the rate-controlling step. The hydrolysis reactions of N-salicylidene-and Nbenzylidenebenzoylhydrazines have been studied for comparison.Paper 1 /02437J

1992 chemical equilibria, complexation equilibria chemical equilibria, complexation equilibria L 5000 18 -051 Biologically Active Acylhydrazones. Part 1. Acid-Base Equilibria and Acid Hydrolysis of Pyridoxal Aroylhydrazones and Related Compounds. -In contrast to the methanolic solution, the tautomer (II) of the title compounds is predominant in aqueous solution. Protonation proceeds in 3 successive steps which are affected by the aryl group. The equilibria are studied at pH values above 3. There, hydrolytic cleavage of the C=N linkage takes place. -(LEES-GAYED, N. J.; ABOU-TALEB, M. A.; EL-BITASH, I. A.; ISKANDER, M. F.; J. Chem. Soc., Perkin Trans. II (1992) 2, 213-217; Chem. Dep., Fac. Sci., Alexandria Univ., Alexandria, Egypt; EN)