Solvation and hydrophobic hydration of alkyl-substituted ureas and amides in NN-dimethylformamide + water mixtures

Abstract

Enthalpies of solution of five alkyl-substituted ureas and seven different amides have been determined at 298.15 K in mixtures of NN-dimethylformamide (DMF) and H₂O. Methyl substitution of the ureas causes changes in the enthalpies of transfer from H₂O to DMF which show that either side of these molecules is solvated independently. From the measurements on the amides it is concluded that methyl substitution at the N atom gives changes in the enthalpies of transfer from H₂O to DMF which are different from those caused by methyl substitution at the C atom. Analysis of the data in the mixed solvent shows that introduction of more or longer alkyl groups into the molecules makes both ureas and amides considerably more hydrophobic. After accounting for the influence of the NH protons, in both ureas and amides, the enthalpic effect of hydrophobic hydration of the solutes was calculated by application of a clathrate-like hydration model. Enthalpic effect of N-substituted methyl groups in ureas and amides prove to be virtually equal. The variation in the enthalpic effects of hydrophobic hydration with the number of C atoms in the n-alkyl group is comparable to that found for alcohols and amines.