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

The proton transfer equilibrium constants (KPT) for 1 : 1 complex formation between Chloranilic Acid (CA) and a series of p -a ndm-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 KPT 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 KPT and -Hammett substituent constants, or pKa values for m -a ndp-substituted 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.