Calorimetry as a tool for the determination of the intramolecular hydrogen bond enthalpy in ortho-substituted phenols

M. Varfolomeev1, D. Abaidullina1 and B. Solomonov1

1Kazan (Volga region) Federal University, Russia

Keywords: intramolecular hydrogen bond
property: enthalpy of solution, enthalpy of vaporization, enthalpy of formation
material: ortho-substituted phenol

Substituted phenols are the structural fragments of different biologically active molecules. It is known that reactivity of phenols in solution depends on three factors: hydrogen bond formation, structure and position of substituents. The formation of intramolecular hydrogen bonds has a very pronounced effect on the physical-chemical properties of ortho-substituted phenols. A large variety of such compounds has been identified as natural and synthetic antioxidants and food additives. In comparison with the meta- and para-isomers, ortho-substituted phenols have a lower dissociation energy of O-H bond due to the intramolecular hydrogen bonding. In present work, two additional ways to derive the intramolecular hydrogen bond strength were suggested using calorimetric methods.

First way is based on thermochemical measurements of gaseous enthalpies of formation of a series of ortho-substituted phenols and its methyl derivatives. In order to get quantitative information on the strength of the hydrogen bonding, thermochemical measurements (combustion calorimetry and vapor pressure measurements) have been performed. From these data the enthalpies of formation of studied compounds in gas phase have been obtained. The values of the pairwise interactions of substituents (OH and CH3O) in the benzene ring as well as the strength of the hydrogen bonds have been derived using these results.

Second way to derive strength of the intramolecular hydrogen bonds in ortho-substituted phenols is based on a thermodynamics of solvation of phenols in proton acceptor solvents. It is known that the enthalpy of solvation of solute A in solvent S is a measure of solute-solvent non-covalent interactions. The process to derive the enthalpies of specific interaction of ortho-substituted phenols considers two steps: breaking of the intramolecular hydrogen bonds and the consequent formation of the intermolecular hydrogen bonds. If enthalpies of specific interaction and intermolecular hydrogen bond are known, strength of intra-HB can be calculated.

Enthalpies of intramolecular hydrogen bonds in few substituted phenols were determined. Obtained data were compared with the known literature values. Some regularities between the structure of studied molecules and its ability to hydrogen bonding were discussed.

This work was supported by Russian federal program "Scientific and scientific-pedagogical personnel of innovative Russia" (N P1349).

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