Azo-hydrazone tautomerism of phenylazonaphthol sulfonates and their analysis using the semiempirical molecular orbital PM5 method

Abstract

The enthalpy of formation for the azo and hydrazone tautomers (A & H Ts) of 1-phenylazo-2-naphthols, 2- and 4-phenylazo-1-naphthols and 4-phenylazo-1-naphthol-2-sulfonic acids was estimated both in the gaseous phase and in water using a semi-empirical molecular orbital PM5 and COSMO method. Substituent effects on the solvation energies for these phenylazonaphthols were analysed in detail. The A & H Ts of azo dyes have small enthalpies of transformation which are the same as heterocyclic tautomers in heterocyclic compounds such as nucleic acid bases and heterocycles of amino acids. The hydroxyazo dyes examined exist mostly as the hydrazone tautomer (HT) in water, since the HTs possess usually larger (minus) values of solvation energy than the azo tautomers (ATs). Dyes whose A & HTs possess comparable energy of solvation result in higher stability of the ATs in water than the HTs since the ATs are more stable in the gas phase. Sulfonic acid groups ortho ( o) to the azo group have unusual solvation energies resulting in dyes whose ATs have higher stability than the HT's in water. Examples of such azo dyes examined are the 4- and 2-( o-sulfophenylazo)-1-naphthols and their derivatives. In general, electron-withdrawing substituents have a small effect on the azo-hydrazone tautomerism (AHT) of phenylazonaphthols with the exception for sulfonic acid groups at certain positions in the structures. Examples of commercial reactive azo dyes showing such a tendency are C.I. Reactive Red 1, Red 194 and Red 227. Chloride substituents in the triazine ring imparted strong effects on the AHT of C.I. Reactive Red 1.