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: An alkene is one of the three classes of unsaturated hydrocarbons that contain at least one carbon- carbon double bond and have the general molecular formula of CnH2n (the other two being alkynes and arenes). ... Hydroxide is a functional group consisting of oxygen and hydrogen: -O−H It has a charge of 1-. The term hydroxyl group is used when the functional group -OH is counted as a substituent of an organic compound. ... Covalent bonding is a form of chemical bonding characterized by the sharing of one or more pairs of electrons between atoms, in order to produce a mutual attraction, which holds the resultant molecule together. ... In chemistry, a carbonyl group is a functional group composed of an atom of carbon double-bonded to an atom of oxygen. ... A ketone is either the functional group characterized by a carbonyl group linked to two other carbon atoms or a compound that contains this functional group. ... An aldehyde is either a functional group consisting of a terminal carbonyl group, or a compound containing a terminal carbonyl group. ... In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently. ... In organic chemistry, a keto-enol tautomerism is an equilibrium between a ketone or aldehyde (the keto form) and an enol. ...
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). In general usage, alcohol (from Arabic al-khwl الكحول, or al-ghawl الغول) refers almost always to ethanol, also known as grain alcohol, and often to any beverage that contains ethanol (see alcoholic beverage). ...
Enolate ion
When the hydroxyl group (−OH) in an enol loses a hydrogen ion (H+), a negative enolate ion is formed as shown here:
A carbonyl compound with an alpha-hydrogen readily undergoes abstraction by a nucleophile to produce the enolate anion/carbanion.
The reactions we discussed involve the carbanion form as the reactive form, considering the reactions involve this alpha-carbon attacking the carbonyl of another molecule.
The carbonyl is reformed as the leaving group is identical to the initial attacking nucleophile, the alkoxide anion..
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.
Lithium enolate derivatives of aldehydes or ketones may be formed at low temperature by slow addition of the carbonyl compound to an excess of LDA in THF (-78 ºC).
The dominant enolborinate in all cases is that derived from the kinetically favored enolate anion.
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