The increasing tightness of fully associated states as a function of their increasing stability. The dimerisation of carboxylic acids

Abstract

Literature data describing the dilution of carboxylic acids in cyclohexane are reinterpreted according to a monomer–dimer equilibrium (higher order oligomers had previously been postulated in order to explain the data). Apparent dimerisation constants for each of the acids were calculated by incorporating a factor to take into account the varying relative permittivity of the solution during the titration. Dilution titrations of propionic acid using diluents of varying relative permittivities were also carried out and apparent dimerisation constants calculated in a similar manner. The dimerisation constants were found to vary according to the relative permittivity of the diluent, and the tightness of the dimers formed (as evidenced by the chemical shift of the carboxy proton) also varied according to the diluent relative permittivity. The results demonstrate that complex stability is directly related to the strength of the hydrogen bonds which go to form that complex. They also clarify a long-standing debate regarding the origin of an anomalous downfield shift of the carboxylic acid proton seen on dilution of simple aliphatic carboxylic acids with cyclohexane.