Effect of Hetero Atom on the Hammett’s Reaction Constant ( ) from the Physical Basis of Dissociation Equilibriums of (Dithio) Benzoic Acids and (Thio) Phenols and Its Application to Solvolysis Reactions and Some Free Radical Reactions

The emergence of putative Hammett equation in mid 1930s was a boon to physical-organic chemists to elucidate the reaction mechanisms of several organic reactions. Based on the concept of this equation several hundreds of papers have emerged in chemical literature in the last century on the effect of structure, on reactivity, and very few on thermodynamic stability and kinetic reactivity of intermediates. In this article an attempt is made to explain the effect of hetero atom on Hammett’s reaction constant (ρ) taking the dissociation equilibriums of benzoic acids, dithiobenzoic acids, phenols, and thiophenols. 1. Introduction Ever since the Hammett equation was developed [1, 2], there were several hundreds of redox, condensation, disproportionation, nucleophilic and electrophilic substitution, and addition reactions with meta- and para-substituted benzene derivatives in the literature, for which the Hammett reaction ( ) constants were reported. Inclusion of those references here is beyond the scope this article as they run into several pages. However the readers can find many articles and reviews in several standard physical-organic chemistry text books. In addition to these numerous reactions, a few reactions were reported by one of the authors (V. Jaganndham) from elsewhere [3] and from our laboratory [4, 5] on the solvolysis and reactions of intermediate carbocations with nucleophilic solvent water. An effect of α-hetero atom substitution on kinetic and thermodynamic stability of intermediate carbocations were also reported from elsewhere [6, 7] and from our laboratory [8]. But in these reactions [3–5] no attempt is made to explain the effect of α-hetero atom on the Hammett reaction constant ( ), which we tried to explain in the present work taking the title equilibriums as staple examples. 2. Results and Discussion The effect of substituent either in meta- or para-position in the benzene ring on the rate or equilibrium constant is given by Hammett [2] in the form of a formula: where is the equilibrium or rate constant of the substituted reactant, is that of unsubstituted reactant, “ ” is the free energy change for equilibrium process or rate process, “ ” is the distance between the substituent and the reaction center, “ ” is the dielectric constant of the medium, and , and are the constants. Here depends on the substituent and and depend on the nature of the reaction. Later, based on some experimental observations Hammett rearranged (1) to the form: where and , (2) is now known as famous Hammett equation. The magnitude of depends on the substituent