text: bonds adjacent to the carbonyl in ketones are more acidic pK ≈ 20) than the  bonds in alkane (pK ≈ 50). This difference reflects resonance stabilization of the enolate ion that is formed upon deprotonation. The relative acidity of the α-hydrogen is important in the enolization reactions of ketones and other carbonyl compounds. The acidity of the α-hydrogen also allows ketones and other carbonyl compounds to react as nucleophiles at that position, with either stoichiometric and catalytic base. Using very strong bases like lithium diisopropylamide (LDA, pK of conjugate acid ~36) under non-equilibrating conditions (–78 °C, 1.1 equiv LDA in THF, ketone added to base), the less-substituted kinetic enolate is generated selectively, while conditions that allow for equilibration (higher temperature, base added to ketone, using weak or insoluble bases, e.g., sodium ethoxide| in ethanol|, or NaH) provides the more-substituted thermodynamic enolate.
  Ketones are also weak bases, undergoing protonation on the carbonyl oxygen in the presence of Brønsted acids. Ketonium ions (i.e., protonated ketones) are strong acids, with pK 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 pK of 5.2 are able to serve as catalysts in this context, despite the highly unfavorable equilibrium constant for protonation (K ).