A 13C nuclear magnetic resonance study of aliphatic ketone protonation in sulfuric acid solutions

A 13C nmr investigation of the protonation in sulfuric acid of the aliphatic ketones MeCOR (R = Me, Et, i-Pr, tert-Bu), 3-pentanone, and the cyclic ketones from cyclobutanone through cyclooctanone is reported. With the exception of cyclobutanone which is considerably less basic, these ketones are all very similar in their basicity, with half-protonation in the region 73–78% H2SO4. The acidity dependence of the protonation is discussed; there appears to be a small dependence on molecular size. 13C nmr is shown to be a suitable alternative to 1H nmr in the study of the protonation of weak organic bases in strongly acidic solutions.