KINETICS AND MECHANISM OF OS(VIII)-CATALYZED OXIDATION OF SOME SUBSTITUTED TRANS-CINNAMIC ACIDS BY CHLORAMINE-T IN ALKALINE-MEDIUM - A NONLINEAR HAMMETT PLOT

The kinetics and mechanism of Os(VIII) catalysed oxidation of some: su bstituted trans-cinnamic acids by chloramine-T in presence of alkaline medium has been investigated. The order with respect to [CAT] is zero and is [OsO4] one. The reaction is fractional order in [substrate]. T he rate dependance on [OH] is inverse first order. Both the electron-r eleasing and withdrawing substituents Facilitate the reaction rate. Th e observed rate constants for substituents are plotted against Hammett constant sigma, a non-linear, 'V' shaped curve is obtained. The elect ron-withdrawing substituents fall on one side of the curve having a ne gative rho value, and electron-releasing substituents fall on the othe rside with positive rho value. The break in the Hammett plot is due to change in the nature of transition state. The mechanism proposed and the derived rate law are in confirmity with the observed results.