18-1 Acidity of Aldehvdes and Ketones:Enolate lons CHAPTER 18 g水 &8le18m8rnemol9tesahwdroen Enols,Enolates and the Aldol Condensation: a,B-Unsaturated Aldehydes and Ketones .. Y wow 18-2 Keto-Enol Equilibria eto-enol to gve two atiantcgomehmengoooom0tenreygeaRo 地8 noonn Substituents can shift the keto-enol equilibrium. todeuterium exchange and oorothe om dr 1 1 CHAPTER 18 Enols, Enolates and the Aldol Condensation: α,β-Unsaturated Aldehydes and Ketones 18-1 Acidity of Aldehydes and Ketones: Enolate Ions The pKa values of the α-hydrogens of aldehydes and ketones range from 16 to 21, comparable to those of alcohols (15-18). Strong bases can remove α hydrogens leading to anions called enolate ions or enolates. The enolate resonance hybrid possesses partial negative charges on both carbon and oxygen and may attack electrophiles at either position. A species that can attack at two different sites to give two different products is called ambident. Alkylation of cyclohexanone enolate with 3-chloropropene occurs at the carbon atom, while protonation occurs at the oxygen atom. 18-2 Keto-Enol Equilibria An enol equilibrates with its keto form in acidic or basic solution. Keto-enol tautomerism is the interconversion of the thermodynamically more stable keto form and the enol form of a carbonyl compound. Keto-enol tautomerism proceeds by either base or acid catalysis. Either equilibration is fast and reversible in solution in the presence of the required catalyst. Substituents can shift the keto-enol equilibrium. For ordinary aldehydes and ketones, only traces of the enol form are present. The enol form is less stable by 8-12 kcal mol-1. However, for acetaldehyde, the enol form is about 100-times more stable than that of acetone because the less substituted aldehyde carbonyl is more stable than the more substituted ketone carbonyl. Enol formation leads to deuterium exchange and stereoisomerization. Treatment of a ketone with traces of acid or base in D2O solvent leads to the exchange of all the hydrogen atoms in α carbons: The number of α hydrogens in a molecule can be readily determined by following the disappearance of the 1H NMR signal as the hydrogens are sequentially replaced by deuterium