Kinetics and Mechanism for Oxidation of L ‐Proline by Bis(hydrogen periodato)argentate(III) Complex Anion

Oxidation of L ‐proline by bis(hydrogen periodato)argentate(III) complex anion, [Ag(HIO 6 ) 2 ] 5− , has been studied in aqueous alkaline medium in the temperature range of 25–40 °C by use of conventional spectrophotometry. Under the conditions for kinetic measurements, the oxidation results in decarboxylation of proline, giving rise to γ ‐aminobutyrate as identified by mass spectrometry. The oxidation kinetics is first order with respect to the silver(III) and proline concentrations; the second‐order rate constants, decreasing with increasing [periodate], are essentially independent of [OH − ]. The kinetic results were interpreted in terms of a reaction mechanism which involves a pre‐equilibrium between [Ag(HIO 6 ) 2 ] 5− and [Ag(HIO 6 )(H 2 O)(OH)] 2− , a mono‐periodate coordinated silver(III) complex. Both Ag(III) complexes are reduced parallelly by the fully deprotonated form of proline in rate‐determining steps (described by k 1 for the former Ag(III) and k 2 for the latter one). The determined rate constants and their associated activation parameters are k 1 (25 °C) = (1.87±0.04) L·mol −1 ·s −1 , Δ H 1 ≠ = (45±4) kJ· mol −1 , Δ S 1 ≠ = (−90±13) J·K −1 ·mol −1 and k 2 (25 °C) = (3.2±0.5) L·mol −1 ·s −1 , Δ H 2 ≠ = (34±2) kJ·mol −1 , Δ S 2 ≠ = (−122±10) J·K −1 ·mol −1 . This is also the first time to find that [Ag(HIO 6 ) 2 ] 5− is also reactive toward a particular reductant and shows a lower reactivity than [Ag(HIO 6 )(OH)(H 2 O)] 2− .