Enols (also known as alkenols) are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond. Enols and carbonyl compounds (such as ketones and aldehydes) are in fact isomers; this is called keto-enol tautomerism:
The enol form is shown on the left. It is usually unstable, does not survive long, and changes into the keto (ketone) form shown on the right. This is because oxygen is more electronegative than carbon and thus forms stronger multiple bonds. Hence, a carbon-oxygen (carbonyl) double bond is more than twice as strong as a carbon-oxygen single bond, but a carbon-carbon double bond is weaker than two carbon-carbon single bonds.
Only in 1,3-dicarbonyl and 1,3,5-tricarbonyl compounds does the (mono)enol form predominate. This is because the resonance and intermolecular hydrogen bonding that occurs in the enol form is not possible in the keto form. Thus, at equilibrium, over 99% of propanedial (OHCCH2CHO) molecules exist as the monoenol. The percentage is lower for 1,3-aldehyde ketones and diketones. Enols (and enolates) are important intermediates in many organic reactions.
When the hydroxyl group (−OH) in an enol loses a hydrogen ion (H+), a negative enolate ion is formed as shown here:
Enolates can exist in quantitative amounts in strictly Brønsted acid free conditions, since they are generally very basic.
1,3-dicarbonyl and 1,3,5-tricarbonyl compounds are quite acidic because of the strong resonance stabilization created when one of the hydrogens is removed (from either the keto or enol forms). The resonance of the enol is exactly analogous to that used to explain the acidity of phenols and consists of the delocalisation of the enolate ion's negative charge to the alpha carbon. These enolate ions are very valuable in synthesis of complicated alcohols and carbonyl compounds (aldol additions). The synthetic value is due to the nucleophilicity of α-carbon of enolate group.
In ketones (a type of carbonyl) with acidic α-hydrogens on either side of the carbonyl carbon, selectivity of deprotonation may be achieved to generate the enolate directly from the ketone. At low temperatures (-78°C, i.e. dry ice bath), in aprotic solvents, and with bulky non-equilibrating bases (e.g. LDA) the "kinetic" proton may be removed. The "kinetic" proton is the one which is sterically most accessible. Under thermodynamic conditions (warmer temperatures, weak base, and protic solvent) equilibrium is established between the ketone and the two possible enolates, the enolate favoured is termed the "thermodynamic" enolate and is favoured because of its lower energy level than the other possible enolate. Thus, by choosing the "correct" conditions to generate an enolate, one can increase the yield of the desired product while minimizing formation of undesired products.