Solvent Hydrogen Bonding and Structural Effects on Nucleophilic Substitution Reactions: Part 5 – Reaction of 2-Bromo-5-nitropyridine with para-Substituted Anilines in Acetonitrile/Dimethylformamide Mixtures

Abstract

Substitution reactions of some para-substituted anilines with 2-bromo-5-nitropyridine are carried out conductometrically in binary acetonitrile/dimethylformamide mixtures. The second-order rate constants correlate well with Hammett’s substituent constants yielding a negative value of ρ. The multiparameter correlation with Kamlet-Taft’s solvatochromic parameters is excellent (100R2 ~ 98%), and the computed percentage contributions of these parameters (Pα = 58%, Pβ = 4%, Pπ∗ = 38%) suggest that both specific and non-specific solute-solvent-solvent interactions influence the reactivity. The solvation model proposed is well supported by the solvatochromism exhibited by the anilines in the solvent mixture under investigation. The molar extinction coefficient (εmax) of aniline varies appreciably up to ~ 25% with a change in the mole fraction of the mixture. The multivariate correlation analysis of εmax (with α, β , π∗) suggests that the solvation around the NH2 moiety of aniline through the solvent’s hydrogen-bond donor (HBD) property is found to be dominant in the solvation process and consequently in altering the rate. The dominance of the solvent’s HBD property in solvation is further confirmed by the cyclic voltammetric oxidation of aniline in the solvent mixture.