Kinetics and mechanism of oxidation of N-methylethylamine by bis(hydrogenperiodato)argentate(III) complex anion

Abstract

Oxidation of N-methylethylamine by bis(hydrogenperiodato)argentate(III) ([Ag(HIO6)2]5−) in alkaline medium results in demethylation, giving rise to formaldehyde and ethylamine as the oxidation products. The oxidation kinetics has been followed spectrophotometrically in the temperature range of 20.0–35.0 °C, and shows an overall second-order character: being first-order with respect to both Ag(III) and N-methylethylamine. The observed second-order rate constants k′ increase with increasing [OH−] of the reaction medium, but decrease with increasing the total concentration of periodate. An empirical rate expression for k′ has been derived as: k′ = (k a + k b[OH−])K 1/{f([OH−])[IO4 −]tot + K 1}, where k a and k b are rate parameters, and K 1 is an equilibrium constant. These parameters have been evaluated at all the temperatures studied, enabling calculation of activation parameters. A reaction mechanism is suggested to involve two pre-equilibria, leading to formation of an intermediate Ag(III) complex, namely [Ag(HIO6)(OH)(MeNHEt)]2−. In the subsequent rate-determining steps, this intermediate undergoes inner-sphere electron transfer from the coordinated amine to the metal center via two distinct routes, one of which is spontaneous while the other is mediated by a hydroxide ion.