Oxidation of Phenylalanine using Rhodium (iii)-catalyst by bromate in acidic medium

Abstract: The kinetics of the rhodium(III)-catalyzed oxidation of phenylalanine in an acidified KBrO₃ solution, utilizing Hg(OAc)₂ as a scavenger, were systematically investigated across a temperature range of 30–45 °C. The reaction demonstrated first-order dependence on Rh(III) concentration, confirming its pivotal catalytic role, while exhibiting first-order kinetics with respect to bromate, implying that bromate does directly influence the rate-determining step. The order of reaction with respect to each reactant was determined by varying the concentration of oxidant, substrate and Rh(III) .The reaction shows negligible effect of [Hg(OAc)2],[H+] and ionic strength of the medium. Chloride ion positively influenced the rate of reaction and mercuric acetate one by one in different sets keeping the concentration of all other reaction constant temperature 35⁰C. A plausible multi-step mechanism was proposed, involving (i) the formation of a Rh(III)-phenylalanine coordination complex, (ii) its rate-limiting oxidation to generate reactive intermediates, and (iii) subsequent fast steps yielding final products. The derived rate law, consistent with experimental data, emphasized the dominance of the Rh(III)-phenylalanine interaction in controlling the reaction kinetics. Thermodynamic parameters (activation energy Ea​, enthalpy ΔH, entropy ΔS) were calculated from Arrhenius and Eyring plots using rate constants at 30–45 °C, revealing insights into the energy barrier and transition state structure. The negative ΔS values suggested a structured associative mechanism, while ΔH highlighted the energy demands of bond reorganization. These findings collectively elucidate the intricate interplay of catalyst-substrate coordination, acid-base equilibria, and intermediate stability in governing the oxidation dynamics of phenylalanine under acidic conditions.

Keywords: bromate, catalysis, oxidation, rhodium (III), acidified potassium bromate.