Enol

From Freepedia

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

Enolate ion

When the hydroxyl group (−OH) in an enol loses a hydrogen ion (H⁺), a negative enolate ion is formed as shown here:

1.3 dicarbonyl and 1,3,5 tricarbonyl compounds are quite acidic because of the strong resonance stabilisation created when one of the hydrogens is removed (from either the keto or enol forms). The resonance from the enol is exactly analgous 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).