Studies on the effect of substituents on the solvational behavior of 2-, 3- and 4-benzoic acids and their anions in ethanol+ water mixtures from conductometric and solubility measurements

Department of Chemistry, Vivekananda Mission Mahavidyalaya, Viveknagar, Chaitanyapur (Haldia)-721 645, India Department of Chemistry, Kalyani University, Kalyani-741 235, India Manuscript received 11 September 2002 The solvational behaviour of 2-, 3- and 4-substituted-chloro, -bromo, -iodo and -nitrobenzoic acids were examined from the solubility of the acids and the dissociation constants for the reaction, HA ⇌ H+ + A- (where HA is substituted benzoic acid) determined conductometrically using Fuoss-Kraus method in ethanol+ water mixtures (0-87 wt % of ethanol). The solubility values increase with the increase in hydrophobic character of the solvent mixtures and the changes depend both on the hydrophobic character and the dielectric constants of the solvent media. The sequence of the solubilities of the substituted acids are in the order : 2-compounds > 3-compounds > 4-compounds and -N02 > -Cl > -Br > -1. The A0-values of the substituted benzoic acids are almost similar but the order is A0 (ortho) > A0 (meta) > A0 (para). A0-values of the nitrobenzoic acids are lower but the order is reversed. The Gibbs energies of transfer of anions were determined from the relation, Δ G0t(dissn.) = Δ G0t (H+) + Δ G0t (A-) - ΔG0t(HA) using the previously determined values of ΔG0t(H+). The ΔG0t(A-) values are found to be increasingly positive in aquo-organic mixtures and ΔG0t(A-) values are usually in the order: 2-compounds > 3-compounds > 4-compounds. Nitrobenzoates differ from halobenzoates in their solvational behavior. The results are discussed in the light of change in the solute-solvent interactions of the substituted acids due to the change in dipole moments, polarizability, hydrogen-bonding and dispersion interactions. The effects of substitution on the interaction energies of anions were determined. Substituents change the interaction energies of neutral acids and their anions in a different way