Proton chemical shifts and thermodynamics of the formation of hydrogen-bonded dimers and mixed 1:1 associates in the ternary system acetic acid/methanol/tetrahydrofuran-d8

Abstract

The chemical shifts of the carboxylic proton of acetic acid and of the hydroxylic proton of methanol dissolved together and separately in tetrahydrofuran-d8 were determined as a function of the temperature and the solute concentrations. The determination of these chemical shifts was possible because in very pure samples the line positions are not affected by proton-exchange reactions. The data can only be explained by the presence of the following association reactions between hydrogen-bonded species which involve the solvent, S: RCOOH-*S + RCOOH-43 RCOOH-*RCOOH--S + S RCOOH4 + ROH4 RCOOH*-ROH.-S + S ROH.43 t RCOOH4 + ROH-**RCOOH***S + S ROH4 + ROH43 ROH*-ROH.-S + S The reaction enthalpies are given by $4.2, -1.4, -3.0, and -3.3 kJ mol-', the reaction entropies at 298 K by +10.3, -3, -9, and -16 J K-l mol-l. These data are not very far from zero, indicating that the number of hydrogen bonds remains constant during the association. Nevertheless, the extent of association is very low, and the quasi monomers which are hydrogen bonded to the solvent dominate. This arises from the high concentration, cs, of the free solvent. The change of cs at higher solute concentrations has to be taken into account in the treatment of the experimental data. Similarly, hydrogen bonding between the donors and the solvent is the reason that cyclic 1:l associates and higher associates are not observed and the reason for the slow proton exchange. It is shown that the thermodynamic data of proton donor association in different media obey linear enthalpy-entropy relationships which are typical for the functional proton donor group.