kinetics and mechanism of the reaction between 3-methylbenzenediazonium ions and catechol

We have investigated the kinetics and mechanism of the reaction between 3-methylbenzenediazonium, 3MBD, ions and catechol (1,2-dihydroxybenzene, [CAT]) in aqueous buffer solution under acidic conditions by employing spectrometric UV/VIS. The variation of the observed rate constant, k_obs, with the acidity at a given [CAT] follows an upward curve, with rate enhancements of up to 20-fold on going from pH = 3.0 up to pH = 5.0, suggesting that the reaction takes place with the monoanionic form of [CAT]. At relatively high acidities, the variation of k_obs with [CAT] is linear with an intercept very close to the value for the thermal decomposition of 3MBD; however, a decrease in the acidity leads to saturation kinetics profiles with non-zero pH-dependent intercepts, prompting us to propose a reaction mechanism comprising two consecutive equilibrium processes involving the bimolecular, reversible, formation of a highly unstable Z-diazo ether, which undergoes isomerization to the E-isomer through a unimolecular step. The results highlight the complexity of the reactions of arenediazonium ions with nucleophilic arenes containing two or more hydroxy groups and are valuable to exploit arenediazonium ions as chemical probes to determine antioxidant distributions.