Enthalpies of interaction of polar and nonpolar molecules with aromatic solvents

Abstract

Enthalpies of solution of representative ketones, phenols, alcohols, and ethers have been determined in α,α,α-trifluorotoluene, benzene, toluene, and mesitylene, and combined with heats of vaporization to give enthalpies of transfer from vapor to the aromatic solvents [ΔH(v→S)]. Comparison of these values with ΔH(v→S) of nonpolar model compounds provides an estimate of the special interactions (dipole – induced dipole or charge transfer, and hydrogen bonding) of the polar solutes with each aromatic solvent. Unless the model compound is perfectly matched, an alternative procedure, the pure base method, is superior for evaluating hydrogen bonding.The special interactions of ketones and ethers with aromatic solvents increase with decreasing π electron density of the solvent. By contrast, m-cresol shows the strongest special interaction with the most electron-rich solvent (mesitylene > toluene > benzene > trifluorotoluene). These results demonstrate that —OH and —OR groups undergo quite different interactions, and that a distinctive interaction occurs between the hydroxyl group and the aromatic ring.Calorimetric heats of vaporization have been measured for the solutes n-butyl ether (10.68 ± 0.02 kcal/mol), 2,2,4,4-tetramethylpentane (9.20 ± 0.03), n-pentane (6.36 ± 0.02), 1-butanol (12.46 ± 0.00), and 2-methyl-2-butanol (11.94 + 0.02).