Abstract
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 the 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, A o B o log log
Highlights
Taft equation is often employed in biological chemistry and medicinal chemistry for the development of quantitative structure activity relationships (QSAR)[1]
Sandri and co-workers have used Taft plots to study the polar effects in the aminolysis of β-lactams
The formation of a covalent bond between penicillin derivatives and human serum albumin (HSA) as a result of aminolysis with lysine residues is believed to be involved in penicillin allergies
Summary
Taft equation is often employed in biological chemistry and medicinal chemistry for the development of quantitative structure activity relationships (QSAR)[1]. Sandri and co-workers have used Taft plots to study the polar effects in the aminolysis of β-lactams. The putative Hammett equation, log (K/Ko) = ρσ is useful to explain the effect of structure on reactivity in meta and para substituted benzene derivatives. Using this equation, to evaluate the σ values of various substituents, Hammett assumed a value of 1.00 for ρ at 25o C in aqueous solution for the dissociation equilibriums of benzoic acids[2]. The Taft equation, log (k/ko)B log (k/ko)A = σ*ρ* was useful to explain the effect of structure on reactivity in aliphatic and ortho substituted benzene derivatives. The significance and the basis for this value are discussed in this article
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