Pirketta Scharlin

Ureas and amides as dipolar aprotic solvents in highly basic media. The dependence of kinetic basicity on solvent composition

The basicity of dipolar aprotic solvent–water–HO– systems with amides and ureas as the organic component has been studied kinetically because previous information is not available, excluding some H– values measured for aqueous dimethylformamide (DMF) and tetramethylurea (TMU). It was found that the increase in basicity with the mole fraction of organic component is at least of the same magnitude as in aqueous dimethyl sulfoxide (DMSO). For instance, in the detritiation of chloroform-t the slopes of the plots log(k2/mol–1 dm3 s–1) vs.x(urea) varied between 11.4–14.6 (as compared to 11.0 in aqueous DMSO) when TMU and cyclic ureas, 1,3-dimethylimidazolidin-2-one (DMI) and 1,3-dimethyl-3,4,5,6-tetrahydropyrimidin-2(1H)-one (DMPU), were used as the organic component in solvent mixture. In aqueous TMU acidity functions H– were extrapolated from kinetic results using linear free energy correlations. Agreement with literature values was evident. This method was also used to extrapolate the H– values in aqueous DMPU. On the basis of present work aqueous ureas can be recommended as solvents in highly basic media. The utility of amides, dimethylformamide and dimethylacetamide, is limited by their instability in basic water solutions.