Proton transfer equilibria, temperature and substituent effects on hydrogen bonded complexes between chloranilic acid and anilines

Abstract

The proton transfer equilibrium constants (K_PT) for 1 : 1 complex formation between Chloranilic Acid (CA) and a series of p- and m‒substituted anilines have been measured in 1,4-dioxane spectrophotometrically. The results supported the concept of amine-solvent hydrogen bond formation (short range solvation effect). Beside, this effect, the K_PT values were apparently affected by the electron donation power of the aniline ring substituent, which was transmitted to the interaction center via resonance and inductive effects. Linear relationships between K_PT and σ-Hammett substituent constants, or pK values for m and p anilines,were obtained verifying the above conclusions. The solute-solvent hydrogen bond formation might increase the reactivity of the aniline nitrogen than would the inductive effect of the alkyl group, in case of CA-N-alkyl aniline complexes. The thermodynamic parameters for the proton transfer complex formation were estimated and it was indicated that the solvent–aniline hydrogen bond formation was preferred in the case of p-substituted aniline complexes more than in the case of the corresponding m‒isomer. It has been found that the proton transfer process was enthalpy and entropy controlled.