Enol

Enols (also known as alkenols) are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond. Alkenes with a hydroxyl group on both sides of the double bond are called enediols. Deprotonated anions of enols are called enolates. A reductone is a compound that has an enediol structure with an adjacent carbonyl-group.

The C=C double bond with adjacent alcohol gives enols and enediols their chemical characteristics, by which they present keto-enol tautomerism. In keto-enol tautomerism, enols interconvert with ketones or aldehydes.

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

Keto-enol tautomerism


Enols interconvert with carbonyl compounds that have an α-hydrogen, like ketones and aldehydes. The compound is deprotonated on one side and protonated on another side, whereas a single bond and a double bond are exchanged. This is called keto-enol tautomerism.

The enol form is usually unstable, does not survive long, and changes into the keto (ketone). This is because oxygen is more electronegative than carbon and thus forms stronger bonds.

Tautomerism in multi-carbonyl compounds
In 1,3-dicarbonyl and 1,3,5-tricarbonyl compounds, however, the (mono-)enol form predominates. This is due to intramolecular hydrogen bonding and possibly to an easy internal proton transfer.

Thus, at equilibrium, over 99% of propanedial (OHCCH2CHO) molecules exist as the mono-enol. The percentage is lower for 1,3-aldehyde ketones and diketones (acetylacetone, for example, 80% enol form).



Enolates
When keto-enol tautomerism occurs the keto or enol is deprotonated and an anion, which is called the enolate, is formed as intermediate. Enolates can exist in quantitative amounts in strictly Brønsted acid free conditions, since they are generally very basic. In enolates the anionic charge is delocalized over the oxygen and the carbon . Enolates are somewhat stabilized by this delocalization of the charge over three atoms.



Delocalization
In valence bond theory, this is explained by a phenomenon known as resonance. The two resonance structures described above combine into the resonance hybrid:



While in molecular orbital theory it is represented by three delocalized molecular orbitals, two of them filled:

Selective deprotonation in enolate forming
In ketones with α-hydrogens on both sides of the carbonyl carbon, selectivity of deprotonation may be achieved to generate two different enolate structures. 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 (higher 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 optimal conditions to generate an enolate, one can increase the yield of the desired product while minimizing formation of undesired products.

Enediols
Enediols are alkenes with a hydroxyl group on both sides of the C=C double bond. Enediols are reaction intermediates in the Lobry-de Bruyn-van Ekenstein transformation.



Reductones
Enediols with a carbonyl group adjacent to the enediol group are called reductones. The enediol structure is stabilized by the resonance resulting from the tautomerism with the adjacent carbonyl. Therefore, the chemical equilibrium produces mainly the enediol form rather than the keto form. Reductones are strong reducing agents, thus efficacious antioxidants, and fairly strong acids. Examples of reductones are tartronaldehyde, reductic acid and ascorbic acid.