Kinetics and mechanism of the diazo coupling reaction of arenediazo methyl ethers with β-naphthol. Part 1. The rates of reaction in nonaqueous acid solutions
Abstract
Rate data are presented for the diazo coupling reaction of arenediazo methyl ethers with β-naphthol in solutions of dioxane, ethanol, and propan-2-ol acidified with hydrochloric acid at 25°C. 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 ether. The relationship between the observed rate constants (kobs.) and hydrochloric acid concentration (cHCl) was found to be non-linear. Correlations of reaction rates with the Hammett acidity function (H0) and with the Hammett substituent constant (σx) conform with a mechanism in which the slow step is unimolecular decomposition of the protonated diazo ether, DEH+, to the coupling entity, the diazonium ion, D+, which instantly reacts with β-naphthol to give the azo dye.