 Ketonium ions (i.e., protonated ketones) are strong acids, with p"K"a values estimated to be somewhere between –5 and –7.Although acids encountered in organic chemistry are seldom strong enough to fully protonate ketones, the formation of equilibrium concentrations of protonated ketones is nevertheless an important step in the mechanisms of many common organic reactions, like the formation of an acetal, for example.Acids as weak as pyridinium cation (as found in pyridinium tosylate) with a p"K"a of 5.2 are able to serve as catalysts in this context, despite the highly unfavorable equilibrium constant for protonation ("K"eq < 10−10).