Kinetic analysis of reactions of p-anisidine and N-methyl-p-anisidine cation radicals in acetonitrile using an electron-transfer stopped-flow method

The chemical follow-up reactions after one-electron oxidation of p-anisidine (PA), N-methyl-p-anisidine (MPA), and N-benzyl-p-anisidine (BPA) in acetonitrile (AN) were analyzed with a stopped-flow method utilizing the electron-transfer reaction with the tris(p-bromophenyl)amine (TBPA) cation radical. The reactions of these p-anisidine derivative cation radicals in AN were found to be faster than that of N,N-dimethyl-p-anisidine (DMPA) cation radical, which proceeds via the acid-base interaction between DMPA(.+) and DMPA. In addition, while the rate law of the decay reaction of DMPA(.+) was expressed as-d[DMPA(.+)]/dt = k[DMPA(.+)]-[DMPA], that of PA(.+) has been determined to be-d[PA(.+)]/dt = k[PA(.+)](2)[PA] when PA(.+) was generated in the presence of PA. On the other hand, in the case that PA(.+) was quantitatively formed without PA via equimolar mixing with TBPA(.+), the rate was determined to be -d[PA(.+)]/dt = k[PA(.+)](2). Also, for the reactions of MPA(.+) and BPA(.+), the rate laws were identical to those of PA(.+). From these results, the reaction mechanisms and the effects of the methyl groups on the reaction kinetics of the p-anisidine derivative cation radicals in AN were discussed.