Kinetics and mechanism of the reaction of hydroxopentaaquarhodium(III) ion with L-proline in aqueous solution

Abstract

The kinetics of the reaction of L-proline (H 2 L + ) with hydroxopentaaquarhodium(III) has been studied over the range 2.48 ≤ 10 3 [H 2 L + ] ≤ 7.44, 3.5 ≤ pH ≤ 4.5, 50° ≤ t ≤ 70° at l = 0.1 mol dm -3 (NaClO 4 ). The reaction proceeds through an outersphere association beween [Rh(OH)] 2+ and HL (conjugate base of H 2 L + ) followed by transformation of the outer into an inner sphere complex (chelate is formed) by slow interchange. The anation rate constant k an of RhOH 2+ at 25 °C and I = 0.1 mol dm -3 (NaClO 4 ) is estimated to be 1.56 x 10 -4 s -1 . ΔH≠ and ΔS≠ for k an path are found to be 54.5 ± 3.5 kJ mol -1 and -135.7 ± 10.5 JK -1 mol -1 respectively. The product of the reaction has been isolated and characterised. The substitution reaction follows an Ia path.