THE PROTONATION OF THE CARBONYL GROUP: III. DIARYL KETONES

Abstract

The basicities of 14 diaryl ketones have been determined. Except for 4,4′-bis(dimethyl-amino)benzophenone, which protonates on nitrogen, the pKBH+ values range from −4.19 for 4,4′-dihydroxybenzophenone to −10.12 for 4,4′-dinitrobenzophenone. The basicities are not correlated well by either σ or σ+ substituent constants and it is believed that this is due either to a variation in the degree of rotation (and hence conjugation) of the two aryl rings or to a departure of carbonyl basicities from the conventional acidity scale based on ammonia bases. The effect on the basicity of joining the ortho positions by bridges of various sizes has also been determined and the results are discussed in terms of steric and electronic effects.