Protonation equilibria of some ortho-substituted and annelated aryl and thiophen-2-yl and-3-yl ketones

For some phenyl-(1-8) and thiophen-2-yl (9-11) and thiophen-3-yl (12-14) ke tones quantum-mechanical (PM3) calculations have been performed, and for co mpounds 2, 3, 5-8, 10-14 protonation equilibria have been determined. Pheny l ketones have similar values for the m* parameter and show good linear cor relation between the proton affinities calculated in the gas phase and the measured pK(BH+). values, which in turn parallel the trend for the calculat ed carbonyl-phenyl ring dihedral angle. It appears that the differences in basicity are governed, essentially by "internal" factors (carbonyl-ring con jugation), while the base-conjugate acid differential solvation is not sign ificantly affected by structural changes. In contrast thiophen-2-yl and -3- yl ketones show a complex behaviour, with strong variations in m* values, a nd no linear correlation between proton affinities and pK(BH+). Factors rel ated to the solvent shell organization probably assume an important role he re.