Determination of the pKa of Benzophenones by Capillary Zone Electrophoresis

Electrophoretic behavior of seven benzophenones as a function of the buffer pH was investigated, and their pKa values were determined by capillary zone electrophoresis. The determination of pKa allows us to rationalize the influence of the buffer pH on the migration behavior of benzophenones. The results reveal that both the presence of intramolecular hydrogen bond and the favorable π‐electronic delocalization decrease the degree of the acid dissociation of the hydroxyl groups of hydroxybenzophenones. However, the introduction of a hydroxyl group at the 4‐position or at the 2′‐position of the aromatic ring of hydroxybenzophenones would decrease greatly their pKa1 values. Thus the presence of this type of hydrogen bonding also plays an important role in the acid dissociation of these hydroxybenzophenones.