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Introduction to the Reactions of Enols and Enolates

Reactions of Enols and Enolates

The addition of a nucleophile to their electrophilic carbonyl carbon is a typical reaction of aldehydes and ketones. However, if the carbonyl compound contains an α hydrogen atom, the hydrogen is acidic due to the influence of the carbonyl group. Thus, a base can abstract the α proton of carbonyl compounds, yielding the corresponding α carbanion which is stabilized by resonance with the carbonyl group. In one resonance structure, the negative charge is located at the α carbon, while the double bond is located between the carbonyl carbon and the carbonyl oxygen. In the other resonance structure, the negative charge is located at the carbonyl oxygen, while the double bond is located between the carbonyl carbon and the α carbon. This species is known as enolate anion, or enolate for short. The corresponding neutral species in which the carbonyl oxygen is protonated and the double bond is located between the carbonyl carbon and the α carbon is called enol. A carbonyl compound with an α hydrogen atom is always in equilibrium with its enol. The interconversion between a carbonyl compound and its enol is known as keto-enol tautomerism.

Fig.1
Competition of deprotonation and addition in carbonyl compounds' reactions.

Due to these qualities, carbonyl compounds with an α hydrogen can act as an electrophile, as well as a nucleophile, since they possess both an electrophilic carbonyl carbon and a nucleophilic α carbon. As a result, carbonyl compounds are starting products in a large variety of syntheses.

Due to keto-enol tautomerism, the α hydrogen may be easily exchanged for deuterium. In addition, a chiral carbonyl compound with an asymmetric α carbon can racemize easily. The nucleophilic α carbon of carbonyl compounds can nucleophilically attack carbon electrophiles, such as other carbonyl compounds or alkyl halides. This leads to the formation of new carbon-carbon bonds. Such reactions are of great importance in organic chemistry.

Exercise

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