Reactivity of phenylazonaphthol sulfonates, their estimation by semiempirical molecular orbital PM5 method, and the relation between their reactivity and azo-hydrazone tautomerism

With respect to 4-phenylazo-1-naphthols and 1-phenylazo-2-naphthols, the azo-hydrazone tautomerism (AHT) of the undissociated and dissociated (UD&D) species for their p-sulfo derivatives (Orange I and Orange II) was analysed estimating their enthalpies of formation in both the gaseous phase and water using semiempirical molecular orbital (MO) (PM5 and COSMO) methods. The reactivities of these phenylazonaphthols against oxidizing agents including singlet molecular oxygen were examined using simple frontier orbital theory. The hydrazone tautomers (HT's) of the UD species of the two orange dyes had higher stability in water than the azo tautomers (AT's), while the two tautomers of the D species had similar stability. Both the UD&D species of the dyes resulted in AHT and furthermore both the tautomers of the dyes displayed an acid-base equilibrium. The assumption that the HT's had smaller pK(a) values than the AT's explained some of the experimental results observed. Frontier orbital theory showed that the azo and hydrazone tautomers (A&HT's) for the UD and D species of two orange dyes as well as the A&HT's for phenylazonaphthols with several substituents had similar reactivity towards oxidants in terms of the interaction between HOMO and LUMO, although the positions of reaction changed in the tautomers used. The dissociation of hydroxyazo dyes and/or oxidants changed their mutual reactivity due to a rise in E-HOMO and E-LUMO, although the A&HT's for the D species of orange dyes had almost the same electronic structure as each other, irrespective of the different sites of ionization. (C) 2003 Elsevier Ltd. All rights reserved.