Hydrogen bonding. Part 1.—Equilibrium constants and enthalpies of complexation for monomeric carboxylic acids with N-methylpyrrolidinone in 1,1,1-trichloroethane

A novel calorimetric method has been derived for the simultaneous determination of equilibrium constants and enthalpies of complexation of monomeric carboxylic acids with bases in an inert solvent. The method requires a knowledge of the corresponding equilibrium constants and enthalpies for the monomer/dimer equilibrium in the same solvent. Both sets of K° and ΔH° values have been obtained for a number of carboxylic acids (and some other hydrogen-bond donors) in 1,1,1-trichloroethane at 298 K, using N-methylpyrrolidinone as a standard base. For the first time, it is possible to evaluate the relative hydrogen-bonding strength of monomeric carboxylic acids and other hydrogen-bonding species in an inert solvent. It is shown that unactivated carboxylic acids are no stronger than simple phenols: equilibrium constants for hydrogen bonding towards N-methylpyrrolidinone are acetic acid (109), benzoic acid (118) and phenol (137). It is further shown that the monomeric carboxylic acids are ca. 20 times as strong as the dimeric acids towards N-methylpyrrolidinone (NMP) in 1,1,1-trichloroethane, with respect to the formation of the species RCO2H·NMP in each case.