Trifluoromethyl ketone hydration. Substituent effects of amino-groups and the hydrating properties of aqueous dimethyl sulphoxide

Abstract

The equilibrium constants for hydration of the 4-amino-, 4-methylamino-, and 4-dimethylamino-derivatives of ααα-trifluoroacetophenone have been determined in mixtures of dimethyl sulphoxide (DMSO) and D₂O (or H₂O) using ¹⁹F n.m.r. spectroscopy. The dimethylamino-compound follows the pattern observed earlier for the 4-methoxy-compound, i.e. in mixtures up to 80 mol % DMSO these compounds are more highly hydrated than in pure water. The extent of hydration of the amino- and methylamino-compounds, on the other hand, decreases steadily as the water content of the mixture is decreased. The difference in behaviour of the two groups of compounds is attributed to the presence of acidic protons in the unhydrated forms of the amino- and methylamino-compounds, which are able to form strong hydrogen bonds to DMSO. The results indicate the strong solvent dependence to be expected for σ⁺ substituent constants for the three kinds of amino-group. A sampling of other carbonyl compounds that undergo hydration revealed a considerable variation in the response of such compounds, equilibrium constants to changes in DMSO–water composition, with only one compound, 2-nitrobenzaldehyde, showing the sort of response that is shown by the 4-dimethylamino- and 4-methoxy-derivatives of trifluoroacetophenone.