Proton transfer reactions of 2-alkyl-4-(4-nitrophenylazo)resorcinol monoanions with hydroxide ion in Me2SO–H2O mixtures
The kinetic behaviour observed for removal of the hydrogen-bonded O ⋯ H ⋯ N proton from 2-alkyl-4-(4-nitrophenylazo)resorcinol monoanions by hydroxide ion in Me2SO–H2O mixtures depends on the solvent ratio and on the 2-alkyl substituent (methyl, ethyl or isopropyl). For example, for 2-isopropyl-4-(4-nitrophenylazo)resorcinol monoanion in aqueous solution and in 30%(v/v) Me2SO–H2O the rate goes through a minimum as the hydroxide-ion concentration is increased. In 60 and 80%(v/v) Me2SO–H2O a practically linear dependence of rate on hydroxide-ion concentration is observed whereas in 90%(v/v) Me2SO–H2O the rate increases to a limiting value. The dependence of rate on hydroxide-ion concentration is determined by the relative values of the rate coefficients for attack of hydroxide ion on the hydrogen-bonded and non-hydrogen-bonded forms of the monoanion and the rate coefficients for their interconversion. The effects of the solvent and the 2-alkyl substituent are discussed.