1943-1949The kinetic studies of the nucleophilic
addition of thiophenol (PhSH) to para-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 via 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]/kobs against 1/[PhSH] is linear for
β-nitrostyrene and a plot of 1/kobs
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