Enol

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Enol (or, more officially, but less commonly: alkenol) is an alkene with hydroxyl group on one of the carbon atoms of 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 resonance and intramolecular hydrogen bonding occur in the enol form but are not possible for the keto form. Thus, propanedial (OHCCH₂CHO) exists to an extent of over 99 percent as the monoenol. The proportion is lower for 1,3 aldehyde ketones and diketones. The enol (and enolate) are an important intermediate for many organic reactions.

The words enol and alkenol are combinations of the words alkene (or just -ene, the suffix given to alkenes) and alcohol (which represents the enol's hydroxyl group).

[edit] Enolate ion

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 from the enol is exactly analogous to that used to explain the acidity of phenols and consists of the delocalisation of the negative charge of the enolate ion 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 and minimize the formation of the undesired product.