Kinetics of addition of thiophenol to <i>para-</i>substituted β-nitrostyrenes and β-methyl-β-nitrostyrenes

Abstract

The kinetic studies of the nucleophilic addition of thiophenol (PhSH) to <i>para</i>-substituted derivatives of β-nitrostyrene (βNS) and β-methyl-β-nitrostyrene (βMβNS) have been investigated spectrophotometrically in 50%(v/v) acetonitrile-water at four different temperatures. The reaction follows first order with respect to each substrate and fractional order with respect to thiophenol in both the cases. On the basis of kinetic results, a stepwise mechanism involving the formation of zwitterionic addition complex in an equilibrium step followed by its conversion into product <i>via </i>proton transfer catalytic route by the thiophenol has been proposed for β-nitrostyrene and non-catalytic proton transfer has been proposed for β-methyl-β-nitrostyrene. Based on the rate law derived, a plot of [PhSH]/<i>k</i>_obs against 1/[PhSH] is linear for β-nitrostyrene and a plot of 1/<i>k</i>_obs against 1/[PhSH] is linear for β-methyl-β-nitrostyrene. The effect of solvent composition on the reaction rate indicates

that the increase in the polarity of the medium increases the reaction rate in both reaction series. The study of the effect of substituents in the two reaction series shows that the electron-withdrawing substituents accelerate the reaction rate and the electron-releasing substituents retard it. Good Hammett correlations have been observed for both the series.