Kinetic studies of the deprotonation of bis(phenylazo)resorcinol monoanions by hydroxide ion

Abstract

Interconversion of the intramolecularly hydrogen-bonded monoanions of 2,4-bis(phenylazo)resorcinol, 4,6-bis(phenylazo)resorcinol, and 2-methyl-4,6-bis(phenylazo)resorcinol with the corresponding dianions in the presence of hydroxide ion in aqueous solution or in 20% dioxane–water (v/v) gives complex kinetic behaviour in which the reciprocal relaxation time for the process goes through a minimum value as the hydroxide ion concentration is increased. The results are used to provide a choice between two proposed mechanisms for the similar behaviour of monophenylazoresorcinols. It is argued that the behaviour of monophenylazoresorcinols and bis(phenylazo)resorcinols is compatible with a mechanism for the interconversion of the monoanions and dianions by two simultaneous pathways. One path involves direct attack by hydroxide ion on the hydrogen-bonded proton and in the other path the hydrogen-bonded monoanion is converted into a non-hydrogen-bonded open form from which the proton is removed by hydroxide ion. Values of the rate coefficients for some of these steps are deduced from the kinetic results.