International Journal of Science and Technology

International Journal of Science and Technology>> Volume 7, Number 2, February 2017

International Journal of Science and Technology


Kinetics and Mechanism of Substitution Reaction of Trans-Dichloro-bis-(Ethylenediammine) Cobalt (III) Chloride with Cysteine, Aspartic acid and Phenylalanine

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Author K. Asemave, S.G. Yiase, S. O. Adejo
On Pages 242-247
Volume No. 2
Issue No. 5
Issue Date May, 2012
Publishing Date May, 2012
Keywords kinetics, dissociative mechanism, substitution reaction, cysteine, aspartic acid and phenylalanine


Abstract

Kinetics and mechanistic studies of the substitution reactions of trans-dichloro-bis-(ethylenediammine)cobalt(III) chloride (trans-[CoCl2(en)2]Cl) by cysteine, aspartic acid, and phenylalanine were carried out in aqueous media at ionic strength of 0.01moldm-3 (NaNO3), and at the temperature range of 35oC to 55oC. The amino acids evidently coordinated to the Co(III) complex through the carboxylate group as indicated by the shifts in IR frequencies; 1582.40 1625.39cm-1, 1340.33 1066.41 cm-1; 1645-1641cm-1, 1309-1112 cm-1; and 1555.44-1571cm-1, 1301.29 1050 cm-1 for cysteine, aspartic acid and phenylalanine, and their Co(III) complexes respectively. Stoichiometries of the reactions were also determined as 1:1. That is, one mole of trans-[Co(en)2Cl2]Cl requires one mole of amino acid to give a product. Also the molar conductivity of the complex was obtained as 270 O-1 cm2 mol-1. This showed that trans-[Co(en)2Cl2]Cl can produce a total of three ions per mole. The pseudo- first order plots of ln [At-A8/ A0-A8] vs. time for the reaction are linear. The linearity of the plot is a clear indication that the reaction is first order in trans-[CoCl2(en)2]Cl. Also, plots of log k1 vs. log [Amino acids] show a linear relationship; with the slope of 1.00, explicitly implying that the reaction is first order in [Amino-acid]. Hence, the reaction is second order overall. Therefore the rate equation for the reaction is; d[trans-[CoCl2(en)2]Cl]/dt=k2[trans-[Co(en)2Cl2]Cl][A]. Rate constants k2, of the reactions were obtained as11.40 10-1 0.0739 dm3mol 1s 1, 4.29 10-1 0.0705 dm3mol 1s 1,and 2.97 10-1 0.0337 dm3mol 1s 1 for cysteine, aspartic acid and phenylalanine, respectively. Activation enthalpies and entropies (?H*,and ?S*), were evaluated to be to be 93.32 kJmol-1, 2.94Jmol-1 K-1, 90.74 kJmol-1, 0.14 Jmol-1 K-1, and 96.63 kJmol-1, 15.71 Jmol-1 K-1 for cysteine, aspartic acid, and phenylalanine, respectively . From the values of the activation entropies (?S*), dissociative mechanism is proposed for these substitution reactions. Thus the studies of rate of substitution of trans-[CoCl2(en)2]Cl by these amino provide an insight of complexation of these amino acids and Co(III) centre. The study of substitution reaction between Co(III) complexes with these amino acids give useful information in the development of detoxifying agents using these amino acids especially cysteine.

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