Determination of the activation parameters ∆H≠ and ∆S≠ via a kinetic study of d,l-mandelic acid oxidation by using chromic acid in the presence of 1,10-phenanthroline as a promoter in an aqueous micellar acid medium

Abstract

Chromic acid oxidation of d,l-mandelic acid in the presence and absence of 1,10-phenanthroline (Phen) is studied in an aqueous micellar medium under kinetic conditions, [d,l-mandelic acid] >> [Phen]T >> [Cr(VI)]T at different temperatures. From studies on the effect of temperature on the rate constant (k), the activation parameters ∆H≠ (enthalpy of activation) and ∆S≠ (entropy of activation) are evaluated by using the Eyring equation [−ln (kh/kBT) = ∆H≠/RT − ∆S≠/R]. The high value of ∆H≠ indicates that the phen-catalysed path is favoured mainly due to very high negative value of ∆S≠. The negative value of ∆S≠ and the composite rate constant kcat support the suggested cyclic transition state. Both the catalysed and uncatalysed paths show a first-order dependence on [H+], and both also show a first-order dependence on [d,l-mandelic acid]T and [Cr(VI)]T. The Phen-catalysed path is first order in [Phen]T. These observations remain unaltered in the presence of externally added surfactants. The cationic surfactant N-cetylpyridinium chloride is found to retard the rate of the reaction.