Linear free energy relationship in complex reaction: Tungsten(VI) catalyzed perborate oxidation of S-phenylmercaptoacetic acids

Tungsten(VI) catalyzes perborate oxidation of S-phenylmercaptoacetic acid. The catalyzed oxidation is first order with respect to the oxidant, indepen dent of [H+], and displays Michaelis-Menten dependence on [PhSCH2COOH] and [W(VI)]. The reaction requires time for equilibration between the oxidant a nd the catalyst. Oxodiperoxotungsten(VI) (WO(O-2)(2)) is the probable oxidi zing species, and decomposition of PhSCH2COOH-WO(O-2)(2) complex is rate li miting. At low [PhSCH2COOH] and fixed [W(VI)], the oxidation follows a seco nd order rate law. Operation of the linear free energy relationship in the oxidation has been tested with a Few electron withdrawing and a few electro n donating substituents. At high temperature, electron-withdrawing groups d ecelerate and electron releasing groups accelerate the oxidation, the varia tion in rate with the substituent is in conformity with the Hammett equatio n. But at low temperature, electron donating substituents fail to accelerat e the oxidation. The results are rationalized. (C) 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 675-681, 1999.