The effect of substituents either in meta or in para position in the benzene ring on the rate or equilibrium constant is given by Hammett in the form of an equation: log K = log K o + ρσ or log k = log k o + ρσ respectively. Where K is the equilibrium constant of the substituted reactant and K o is that of un-substituted reactant and k is the rate constant of the substituted reactant and k o is that of un-substituted reactant. ρ is the reaction constant and σ is the substituent constant. From

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... e experimental data of several compounds for the values of K and K o , and choosing an arbitrary value of one for ρ at 25 o C, Hammett computed σ values for several meta and para substituents in benzene derivatives. However the Hammett equation failed to explain the structural effects in ortho-substituted benzene derivatives and aliphatic systems. Subsequently Taft formulated an equation, k to Taft ρ * was given in this article. This is a very useful lecture for graduate students who are interested in learning structure-reactivity correlations and reaction mechanisms.