Steric effects on the proton-transfer equilibria of ketones, sulfoxides, and phenols

The proton-transfer equilibrium of several bases and acids, including some sterically hindered ketones, sulfoxides, and phenols, has been investigated by means of the determination of the thermodynamics of the equilibrium, NM R C-13 relaxation measurements, and quantum chemical calculations. The anal ysis of such data yields information about the steric effects on basicity o r acidity and about the underlying reasons for the anomalous behavior of sp ecies having a sterically hindered basic or acidic site. Thus, it is demons trated that the anomalously low basicity of two sterically hindered ketones (tBu(2)CO and PhCOtBu) is enthalpic in origin and stems from steric hindra nce to the solvation of their protonated forms. No such effect is found for analogous sulfoxides, whereas phenols display a more complex behavior.