Hydrogen bonding. Part 2.—Equilibrium constants and enthalpies of complexation for 72 monomeric hydrogen-bond acids with N-methylpyrrolidinone in 1,1,1-trichloroethane

Abstract

Equilibrium constants and enthalpies of complexation are reported for 72 monomeric hydrogen-bond acids against the base N-methylpyrrolidinone in dilute solutions in 1,1,1-trichloroethane, using a discontinuous calorimetric titration method. Representative oxygen acids (carboxylic acids, phenols, alcohols and oximes) and representative nitrogen acids (anilides, amides, sulphonamides, aromatic amines and heterocyclic amines) are included: carbon acids seem too weak to measure by the present method. There is no general ΔH°vs.ΔG°, nor a general ΔG°vs. pK_a, correlation, but families of acids (e.g. carboxylic acids, phenols and anilides) give rise to linear ΔH°/ΔG° and ΔG°/pK_a correlations. The slopes of the ΔH°vs.ΔG° plots can be positive (substituted alcohols), nearly zero (3- and 4- substituted phenols) or even negative (carboxylic acids and anilides), and a theory invoking solvent participation is advanced to explain this behaviour. It is suggested that the ΔG°(or log K°) values be used to construct a scale of monomeric solute hydrogen-bond acidity, rather than the ΔH° values.