An analysis of azo-hydrazone tautomerism of reactive azobenzene and pyrazolinyl-azo dyes using the semiempirical molecular orbital PM5 method

Abstract

The azo-hydrazone tautomerism (AHT) was analyzed through the enthalpies of formation estimated for ten yellow reactive dyes in their hydrolyzed forms using semiempirical molecular orbital (MO) PM5 and COSMO methods. Introducing a particular substituent at the o-position of an azo group caused a separation of the dyes into azo and hydrazone tautomers according to the group additivity analysis of the AHT. These procedures predicted that AHT in both the gas and water phases occurs as follows: Arylazobenzene dyes with o-amino, o-acetylamino or o-ureido groups existed as the azo tautomers (ATs) in both the gas phase and water with some exceptions in the gas phase, while those with o-hydroxy groups existed as ATs in the gas phase and as hydrazone tautomers (HTs) in water. Due to the keto-enol tautomerism of a pyrazoline ring, three kinds of azo and hydrazone (azo/enol, azo/keto (A/K) and hydrazone/keto) tautomers existed in phenylazopyrazolinyl dyes. Phenylazopyrazolinyl dyes with o-amino groups existed as ATs in both phases, while those with o-hydroxy groups occurred as HTs in the gas phase and as A/KTs in water. Results of the AHT analysis showed that the reason hydroxyazo dyes had higher stability in water was the larger hydration energy of the keto structure than that of the enol one. The AHT and acid–base equilibrium (ABE) of C.I. Acid Yellow 23 were analysed using the same MO methods. The dye existed as A/KTs in water. The A/KTs carried out the ABE, and the deprotonated A/KTs showed another tautomerism. The concurrent picture of AHT and ABE for the dye was consistent with the results reported previously.