Enol

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.

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: