Studies on the composition and kinetics of chromium(III)‐alanine system

Kinetics of the complex formation of chromium(III) with alanine in aqueous medium has been studied at 45, 50, and 55°C, pH 3.3–4.4, and μ = 1 M (KNO 3 ). Under pseudo first‐order conditions the observed rate constant ( k obs ) was found to follow the rate equation:\documentclass{article}\pagestyle{empty}\begin{document}$$ k_{{\rm obs}} = \frac{{(k_{{\rm an}} K_{{\rm IP}} K_a [{\rm H}^{\rm + }] + k'_{{\rm an}} K'_{{\rm IP}} K'_a K_a)[{\rm Alanine}]_T }}{{[{\rm H}^{\rm + }]^2 + [{\rm H}^{\rm + }]K_a + [{\rm H}^{\rm + }]K'_a + K_a K'_a + (K_{{\rm IP}} K_a [{\rm H}^{\rm + }] + K'_{{\rm JP}} K'_a K_a)[{\rm Alanine}]_{\rm T} }} $$\end{document}Values of the rate parameters ( k an , k   an ′ , K IP , and K   IP ′ ) were calculated. Activation parameters for anation rate constants, Δ H ≠ ( k an ) = 25 ± 1 kJ mol −1 , Δ H ≠ ( k   an ′ ) = 91 ± 3 kJ mol −1 , and Δ S ≠ ( k an ) = −244 ± 3 JK −1 mol −1 , Δ S ≠ ( k   an ′ ) = −30 ± 10 JK −1 mol −1 are indicative of an ( I a ) mechanism for k an and ( I d ) mechanism for k   an ′routes (‥substrate Cr(H 2 O)   6 3+is involved in the k route whereas Cr(H 2 O) 5 OH 2+ is involved in k ′ route). Thermodynamic parameters for ion‐pair formation constants are found to be Δ H °( K IP ) = 12 ± 1 kJ mol −1 , Δ H °( K   IP ′ ) = −13 ± 3 kJ mol −1 and Δ S °( K IP ) = 47 ± 2 JK −1 mol −1 , and Δ S °( K   IP ′ ) = 20 ± 9 JK −1 mol −1 .