Kinetics of anation of hexaaquachromium(III) Ion by aspartic acid: mechanism and activation parameters

Summary

The kinetics of the reaction of hexaaquachromium(III) ion with aspartic acid in aqueous-acidic medium have been investigated spectrophotometrically at [H⁺]=1×10⁻⁴ – 10× 10⁻⁴mol dm⁻³, t=35–45 °C and μ=1.0 mol dm⁻³ (KNO₃). The results are in accord with a mechanism involving a fast 1∶1 outer-sphere association between hexaaquachromium(III) ion and the substrate species, followed by their transformation into the product by the process where the incoming ligand is appreciably bounded to the metal ion in the transition state. The rate law consistant with the mechanism is as follows:

$$\begin{gathered} k_{obs} = \frac{{\left( {k_1 K_{IP} K_2 + k_2 K'_{IP} \left[ {H^ + } \right]} \right)\left[ A \right]_T }}{{\left( {{{\left[ {H^ + } \right]^2 } \mathord{\left/ {\vphantom {{\left[ {H^ + } \right]^2 } {K_1 }}} \right. \kern-\nulldelimiterspace} {K_1 }}} \right) + \left[ {H^ + } \right] + K_2 + \left( {K_{IP} K_2 + K'_{IP} \left[ {H^ + } \right]} \right)\left[ A \right]_T }} \hfill \\ A = Aspartic acid \hfill \\ \end{gathered} $$

The rate parameters were evaluated and associated activation parameters calculated using the Eyring equation. Enthalpy and entropy terms for the rate constants:viz. ΔH^≠(k₁)=+81±1 kJ mol⁻¹, ΔH^≠(k₂)=+50±1 and ΔS^≠(k₁)=−54±3 JK⁻¹ mol⁻¹, ΔS^≠(k₂)=−157±2 JK⁻¹ mol⁻¹ are indicative of an (I_a) mechanism. Comparison of the results with published data on this metal ion also provide evidence of an associative interchange mechanism.