The Significance and the Basis for the Taft ρ* Value: A One Hour Physical-Organic Chemistry Classroom Lecture to Graduate Students

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 Ko + ρσ or log k = log ko + ρσ respectively. Where K is the equilibrium constant of the substituted reactant and Ko is that of un-substituted reactant and k is the rate constant of the substituted reactant and ko 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 Ko, and choosing an arbitrary value of one for ρ at 25oC, 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, = ρ*σ*, on the basis of the same grounds of Hammett equation, but using the data on acid and base catalysed hydrolysis of esters. Here k and ko are the rate constants of substituted ester and that of ethyl acetate respectively and the suffixes A and B stand for acid and base catalysis. Taft has taken a value of 2.48 for ρ* that is analogous to the value of one for ρ in the Hammett equation. The basis and significance for the value of 2.48 in the above rearranged equation 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.