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Enolates and Enols Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 16, 2011

Nucleophilic Addition to Conjugated Carbonyl Compounds: The Michael Reaction

α, β –Unsaturated carbonyl compounds can be attacked by nucleophiles at the β carbon, forming an enolate.

Nucleophilic Addition to Conjugated Carbonyl Compounds: The Michael Reaction

Nucleophilic Addition to Conjugated Carbonyl Compounds: The Michael Reaction

Protonation and tautomerization produces a β- alkylated carbonyl compound. These Michael additions, also known as conjugate additions, compete with addition to the carbonyl.

Condensations

A condensation reaction leads to a product with a new C–C bond. Most often the new bond results from a nucleophilic addition of a reasonably stable carbanion-enolate to the C=O group (acceptor) of an aldehyde; less frequently the C=O group belongs to a ketone or acid derivative. Another acceptor is the C≡N group of a nitrile.

Aldol Condensation. The addition of an enolate to the C=O group of its parent compound is called an aldol condensation. The product is a β-hydroxycarbonyl compound. In a mixed aldol condensation, the enolate of an aldehyde or ketone adds to the C=O group of a molecule other than its parent.

Condensations

Aldol condensations are reversible, and with ketones the equilibrium is unfavorable for the condensation product. To carry out condensations of ketones, the product is continuously removed from the basic catalyst. β-Hydroxycarbonyl compounds are readily dehydrated upon heating to give α, β-unsaturated carbonyl compounds.

Condensations

Acylation of Enolates: The Claisen condensation. In a Claisen condensation, the enolate of an ester adds to the C=O group of its parent ester. The addition is followed by loss of the OR group of the ester to give an α, β-ketoester. In a mixed Claisen condensation, the enolate adds to the C=O group of a molecule other than its parent. The sequence of reactions is similar to the reactions discussed above. An enolate is formed, which then adds to the carbonyl of an ester.

Condensations

Next, the carbonyl group reforms, accompanied by the expulsion of the –OR portion of the ester.

Condensations

Each of these steps is reversible. The reaction is driven to completion by an irreversible step, in which a very stable enolate is formed. Acid is then added to protonate the enolate.

Practice problems for these concepts can be found at:  Enolates and Enols Practice Problems

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