Acid-base interactions in the two-component system “carboxylic acid – aprotic solvent”.
DOI:
https://doi.org/10.31558/2617-0876.2017.2.3Keywords:
carboxylic acids, association, self-associates, IR spectroscopy, UV spectroscopy, cryoscopyAbstract
The behavior of carboxylic acids (benzoic, trichloroacetic, acetic) in aprotic solvents in the temperature range 5– 60 °C was studied by methods of UV spectroscopy, cryoscopy, and IR spectroscopy. Analyzing the values of the dimerization constants, one can see that with increase of temperature and the acid properties of carboxylic acids, the amount of dimeric form decreases, and the content of the monomeric form increases due to the equilibrium shift. Additionally, on the ground of the values of the equilibrium constants and the enthalpy of the dimerization, cyclization and addition of the monomer to the linear polymer obtained from the NMR spectroscopy data, it was shown that the equilibrium constant for the formation of a linear dimer from two monomers differs significantly from the monomer addition constant to the linear polymer. The state of the "acetic acid–CCl4" system is mostly affected by cyclic and linear dimerization processes. Comparison of the enthalpy of formation data represents that the strength of the hydrogen bond in the cyclic dimer is much less than in the linear dimer, while the stabilization energy is higher. Also, according to the equilibrium constants of the corresponding process, the amount and stability of the self-associates formed were analyzed: cyclic dimer, linear dimer, linear polymer. Comparison of the values of the equilibrium constant and the enthalpy of the association processes in different media shows that the content of various associated forms and the acid monomer depends on the nature of the solvent. Moreover, the molecules of the solvent can themselves act as bases – a participant of the acid-base processes. For dicarboxylic acids (aliphatic, phthalic) in DMSO and acetonitrile, the strength of the intramolecular hydrogen bond was estimated on the basis of the values of the homoconjugation constants.References
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