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 (k_obs.) and hydrochloric acid concentration (c_HCl) was found to be non-linear. Correlations of reaction rates with the Hammett acidity function (H₀) and with the Hammett substituent constant (σ_x) 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.