Enol


Enol

Enols (also known as alkenols) are alkenes with a hydroxyl group affixed to one of the carbon atoms composing 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 the resonance and intermolecular hydrogen bonding that occurs in the enol form is not possible in the keto form. Thus, at equilibrium, over 99% of propanedial (OHCCH2CHO) molecules exist as the monoenol. The percentage is lower for 1,3-aldehyde ketones and diketones. Enols (and enolates) are important intermediates in many organic reactions.

The words enol and alkenol are portmanteaux of the words alkene (or just -ene, 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:

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 of the enol is exactly analogous to that used to explain the acidity of phenols and consists of the delocalisation of the enolate ion's negative charge 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 while minimizing formation of undesired products.

Natural occurrences

Vitamin C is a sugar acid containing an enol bond. It can lose a proton as pictured, which makes it an acid:

The synthesis of long-chain biomolecules from the two-carbon precursor acetyl CoA is effected by enol chemistry, which allows carbon-carbon bond forming reactions. Fatty acid synthesis consists of sequential additions of the enol of acetyl CoA into an acyl carrier protein-bound carboxylate, until the targeted chain length is attained. In humans, for example, this process effects the formation of fatty acids for fats produced for fat storage in the liver, adipose tissue, and excretion into breast milk.

Another process involving enol chemistry is the mevalonate pathway, which begins by a thiolase-catalyzed enol reaction of acetyl CoA to produce acetoacetyl CoA, and also continues with a HMG-CoA synthase-catalyzed enol reaction of acetyl CoA and acetoacetyl CoA to produce HMG-CoA. After four more reaction steps, isopentenyl pyrophosphate is produced, and one step more gives dimethylallyl pyrophosphate. These intermediates are used for diverse purposes such as biosynthesis of terpenes, terpenoid, and steroids.


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