Base hydrolysis of (αβ S)-(o-methoxy benzoato) (tetraethylenepentamine) cobalt(III) ion: a comparative study of the role of ion pairs and micelles

Abstract

The kinetics of base hydrolysis of (αβ S)-(o-methoxy benzoato) (tetraethylenepentamine)cobalt(III) obeyed the rate law: k_obs = k_OH[OH^-], in the range 0.05 □ [OH^-]_T, mol dm^-3 □ 1.0, I = 1.0 mol dm^-3, and 20.0-40.0°C. At 25°C, k_OH = 13.4 ±0.4 dm³ mol^-1 s^-1, ΔH^≠ = 93 ±2 kJ mol^-1 and ΔS^≠ = 90 ±5 JK^-1 mol^-1. Several anions of varying charge and basicity, CH₃CO₂^-, SO₃^2-, SO₄^2-, CO₃^2-, C₂O₄^2-, CH₂(CO₂)₂^2-, PO₄^3-, and citrate^3- had no effect on the rate while phthalate^2-, NTA^3-, EDTA^4-, and DTPA^5- accelerated the process via formation of the reactive ion pairs. The anionic (SDS), cationic (CTAB), and neutral (Triton X-100) micelles, however, retarded the reaction, the effect being in the order SDS> CTAB > Triton X-100. The importance of electrostatic and hydrophobic effects of the micelles on the selective partitioning of the reactants between the micellar and bulk aqueous pseudo-phases which control the rate are discussed.