Tautomerism of 2-Arylazo-l-naphthols in Various Solventst

Abstract

The tautomerism of 2-(4'-substituted phenylazo)-1-naphthol (2-azo dyes) has been examined by means of electronic spectra in various solvents. With hydrogen and electron-withdrawing groups as the 4'-substituents, the hydrazone form is favored (almost 100%), while with electron-donating groups the azo form is favored to some degrees (0-58%). As examined for the 4'-OCH₃ derivative, the azo form is stabilized in pyridine, acetonp, ethanol, and methanol compared with the equilibrium in benzene, while the hydrazone form predominates in chloroform and acetic acid. Above effects of substituents and solvents have the same tendency to those observed for 4-(4'-substituted phenylazo)-1-naphthol (4-azo dyes). However, for all cases the hydrazone/azo ratios observed for 2-azo dye are much larger than those for 4-azo dye.