Kinetics and mechanism of the diazo coupling reaction of aryldiazo phenyl sulphides with β-naphthol in acid solutions

The diazo coupling reaction of aryldiazo phenyl sulphides with beta-naphthol in solutions of dioxane and ethanol acidified with hydrochloric acid was studied. The kinetics of formation of azo dyes were investigated at 25 degrees C under pseudo-first-order conditions. Over the range of HCl concentrations studied, pseudo-first-order rate constants were obtained and the reaction was found to be of first order in diazo sulphide. The observed rate constants (k(obs)) were found to be independent of acid concentration, and the reaction was only affected by the basicity of the diazo sulphide and the nature of the solvent. Two possibilities for the mechanism of azo dye formation have been deduced and one possible mechanism for this characteristic reaction is supported by the present investigation.