An enol is an alkene that has a hydroxyl group attached to one of the carbon atoms of the double bond. This functional group is called the enol group; although it is more common to use the term enol to refer to alkenols, than the term alkenol itself. Enols and carbonyl groups (such as ketones and aldehydes) are, in fact, isomers; this is what is called keto-enol tautomería:
The enol form is the one shown on the left. It is usually unstable, does not survive much, and changes to the keto form (to the ketone isomer), drawn to the right. This is because oxygen is more electronegative than carbon, and therefore its bond is stronger. A carbon = oxygen double bond is more than twice as strong as a simple carbon-oxygen bond, but a double carbon-carbon bond is not twice as strong as two carbon-carbon single bonds but rather less.
Only in the 1,2-dicarbonyl and 1,3,5-tricarbonyl compounds the (mono) enol form predominates. In this case, it is due to a resonance and an intramolecular hydrogen bridge that has the enol form, but can not have the keto form. Then, the propanodial (O = CH-CH2-CH = O) exists more than 99% as a monoenol.
The word enol derives from alkene (or simply -eno, its suffix) and alcohol (or the suffix -ol). Anión enolato
When an aldehyde loses the Hα (the Hα is the H of the Cα, and this is the one that is neighbor of the C carbonyl), the enolate anion is formed as shown below:
The 1,3 dicarbonyls and 1,3,5 tricarbonyls are slightly acidic, due to the strong resonance stability. These enolate anions are very valuable when synthesizing alcohols and very complex carbonyls, such as Enol (Aldol Addition). The synthetic value is given because it is nucleophilic, because it has a net negative charge.
wiki
