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

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 sigma-Hammett substituent constants, or pK(a) values for m- and p-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.