Solvent hydrogen bonding and structural effects on nucleophilic substitution reactions: Part 4 - Reaction of 2-chloro-5-nitropyridine with para-substituted anilines in acetonitrile/dimethylformamide mixtures

Substitution reactions of some para-substituted anilines with 2-chloro-5-nitropyridine were carried out conductometrically in acetonitrile/dimethylformamide mixtures. The various thermodynamic parameters for the reactions have been reported and discussed along with the validity of the isokinetic relationship. The correlation of second-order rate constants with Hammett's substituent constants yields a fairly linear straight line with negative slope. The rate data correlate satisfactorily (100r2 ~ 93%) with Reichardt's macroscopic solvent parameter, ET(30). While correlation with Kamlet-Taft's solvatochromic parameters is excellent (100R2 ~ 99%) and the computed percentage contribution of these parameters (Pα = 29%, Pβ = 13%, Pπ⁎ = 58%) suggests 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 aniline in the solvent mixture under investigation. The molar extinction coefficient (εmax) of aniline varies appreciably up to ~ 25% with the change in mole fraction of the mixture. The multivariate correlation analysis of ɛmax (with α, β, π⁎) suggests that the solvation around NH2 moiety of aniline through hydrogen bond donor (HBD) property is found to be dominant in the solvation process and consequently in altering the rate. The observation is that the dominance of HBD property in solvation is further confirmed by the cyclic voltammetric oxidation of aniline in the solvent mixture.