Kinetics and mechanism of the reaction of an arenediazonium ion with hydrophilic aminocarboxylic acids in aqueous buffered solution

The reaction of 4‐nitrobenzenediazonium ion, 4NBD, with the aminocarboxylic acids (AA) glycine and serine was studied under acidic conditions by using Linear Sweep Voltammetry (LSV), which allows simultaneous monitoring of 4NBD loss and product formation. Voltammograms of the reaction mixture are complex, showing up to five reduction peaks. The reduction peaks at E_p = ?0.5 and ?1.0 V, not detected in the absence of AA, are associated to products formed in the course of the reaction. The variation of their peak current, i_p, with time shows a complex behavior; that of i_p (E_p = ?1.0 V) follows a biphasic profile with i_p increasing with time up to a maximum after which a decrease is detected, suggestive of formation and subsequent decomposition of a transient intermediate, meanwhile i_p (E_p = ?0.5 V) increases with time after an induction period. The peaks at E_p = ?0.1 and ?0.8 V are associated to the reduction of the diazonium group of 4NBD and, in the presence of AA (AA] >>> 4NBD]), their peak currents decrease exponentially with time following clean first‐order kinetics for more than 3t_1/2. The variation of k_obs with AA] at a given pH is linear with an intercept equal to zero and that of log(k_obs) with pH at constant AA] is also linear. Kinetic evidence is consistent with a reaction mechanism involving an irreversible, rate‐limiting bimolecular step which leads to the formation of an unstable triazene, which further decomposes yielding 4‐nitroaniline among other reaction products. Copyright © 2007 John Wiley & Sons, Ltd.