Kinetics of Oxidation of Substituted Phenols by Potassium Bromate

Kinetic methods of analysis are now accepted as standard analytical procedures for several of reactions, especially oxidation types. Though the reaction of phenols and substituted phenols by potassium bromate is well understood as an oxidative type, there was a need to understand and more evidence required against the participation of Br2 in the increasing rate of reaction with increasing acetic acid composition in the bromate oxidation of phenols. Present work explains the strange behaviour on the basis of formation of HBrO3 that is favoured in acidic medium.

Kinetics of oxidation of 3, 4-Dimethyl phenol, 4-sec-butyl phenol and 4-tert-butyl phenol in aqueous acetic acid – water medium in the temperature range of 308K to 321K were investigated. The reaction obeys first order kinetics with respect to [oxidant], [substrate] and [H+]. The active oxidising species was found to be HBrO3. A suitable mechanism in conformity with the kinetic observation has been proposed. Various activation parameters such as energy of activation (58.59, 61, 43, 65.38 KJ/mol), enthalpy change [56.03, 58.87, 62.82 KJ/mol], free energy change (73.19, 77.17, 77.82 KJ/mol) and entropy of activation (-55.7, -59.45 and -48.78 J/mol/deg) for 3: 4-dimethyl phenol, 4-sec-butyl phenol, 4-tert-butyl phenol at 308K, were obtained. A rate law has been derived on the basis of the data. The order of reactivity among the methyl substituted phenols is 3, 4-dimethyl phenol > p-cresol > phenol. The order of reactivity among higher alkyl substituted phenols is 4-tert-butyl phenol 4-sec-butyl phenol > 4-ethyl phenol >phenol.

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