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

Literature data describing the dilution of carboxylic acids in cyclohexane are reinterpreted according to a monomer-dimer equilibrium (higher order ol igomers had previously been postulated in order to explain the data). Appar ent dimerisation constants for each of the acids were calculated by incorpo rating a factor to take into account the varying relative permittivity of t he solution during the titration. Dilution titrations;of propionic acid usi ng diluents of varying relative permittivities were also carried out and ap parent dimerisation constants calculated in a similar manner. The dimerisat ion constants were found to vary according to the relative permittivity of the diluent, and the tightness of the dimers formed (as evidenced by the ch emical shift of the carboxy proton) also varied according to the diluent re lative permittivity. The results demonstrate that complex stability is dire ctly related to the strength of the hydrogen bonds which go to form that co mplex. 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.