Base catalysed addition of thiophenol to β-nitrostyrene in aqueous acetonitrile - A kinetic study

Research Department of Chemistry, VHNSN College, Virudhunagar-626 001, India E-mail : shunmugee@hotmail.com Manuscript received 11 October 2004 The kinetics of nucleophilic addition of thiophenol (PhSH) to β-nitrostyrene (βNS) has been investigated in 80% (v/v) acetonitrile-water at 25°C. The reaction follows second order, first order in each rcadant. The effect of solvent composition on the rate is significant that the decrease in polarity of the medium decreases the reaction rate. A mechanism involving the formation of zwitterionic addition complex in a fast equilibrium step followed by its conversion into the product by a slow step has been proposed. Evaluation of the equilibrium constant for the formation of zwitterion indicates that the kinetics is not Michaelis-Menten type and the rate law is kobs = k1kc [PhSH]. Kinetics of triethylamine (TEA) catalysed nucleophilic addition of thiophenol to (β-nitrostyrene has also been studied in 80% (v/v) acetonitrile-water at 25°C. The rate of nucleophilic addition reaction has been considerably increased in the presence of added base. The reaction follows first order in [β-nitrostyrene) and [thiophenol] hut fractional order (0.769) in [TEA]. The rate of the reaction is found to increase with decrease in polarity of' the medium. The plot of kobs [PhSH] versus [TEA] gives a straight line with a definite intercept which indicates the presence of non-catalytic route along with catalytic route. On the basis of kinetic results, a stepwise mechanism involving the formation of zwilterion with PhSH and βNS and a complex with the zwitterion and TEA in equilibrium steps followed by their conversion into the product has been inloked. .From the rate law derived both catalytic (kb) and non-catalytic (k) rate constants are evaluated