HYDRAZONE-AZO TAUTOMERISM OF PYRIDONE AZO DYES .3. EFFECT OF DYE STRUCTURE AND SOLVENTS ON THE DISSOCIATION OF PYRIDONE AZO DYES

It has been shown by electronic spectra that hydrazone-azo anion equilibria of arylazopyridones exist in various solvents. The thiadiazoleazopyridones more easily undergo acid dissociation into the azo anion than substituted phenylazopyridones in polar solvents. In alcohols, dimethylformamide, dimethylsulfoxide (DMSO), pyridine, etc., the azo anion predominates, in glacial acetic acid and chloroform the hydrazone predominates; and in acetone, cyclohexanone and benzene the hydrazone and azo anion structures of dyes 1-4 are present in different proportions. Substituted phenylazopyridones (dyes 5-48) favour the hydrazone form in all the solvents used. The concentration of the samples in solutions of ethanol, acetone, cyclohexanone, DMSO, etc. influences the equilibria to a certain extent. The equilibria tend to be shifted to the azo anion side on decreasing the concentration from 10(-4) M to 10(-6) M. Temperature changes within the range 20-80-degrees-C do not exert any obvious influence on the dissociation equilibria. The nature of 3-substituents on the pyridone ring have an effect on pK values, nu(max)hyd and nu(max)anion; the correlation between these parameters and the Hammett-sigma constant of the substituent is discussed.