Properties of ,  -Unsaturated Aldehydes and Ketones 18-8 Conjugated unsaturated aldehydes and ketones are more stable than their unconjugated isomers. Enones, or ,  -unsaturated carbonyl groups, are stabilized by resonance. As a result, acids or bases catalyze a rearrangement of ,  -unsaturated carbonyl compounds to their conjugated  -  -isomers.

,  -Unsaturated aldehydes and ketones undergo the reactions typical of their component functional groups. The conjugated carbonyl group of ,  -unsaturated aldehydes and ketones can undergo reactions involving the entire functional system by: Acid-catalyzed mechanisms Radical mechanisms Nucleophilic addition mechanisms

Conjugate Additions to ,  -Unsaturated Aldehydes and Ketones 18-9 The entire conjugated system takes part in 1,4-additions. Addition reactions involving only one of the two  bonds are called 1,2-additions.

Several reagents add to the conjugated  system in a 1,4-manner. This is called conjugate addition. The nucleophilic part of the reagent attaches to the  -carbon and the electrophilic part (proton) attaches to the carbonyl oxygen. When A is H, the initial product is an enol, which then tautomerizes to its keto form. The end result then appears to be a 1,2-addition.

Oxygen and nitrogen nucleophiles undergo conjugate additions. Conjugate additions of water, alcohols, amines and similar nucleophiles undergo 1,4 additions: These reactions are generally faster and result in higher yields when a base is used as the catalyst. These processes are readily reversed at elevated temperatures.

1,4-products (carbonyl compounds) usually form rather than 1,2-products (hydrates, hemiacetals and hemiaminals) because they are more stable. Exceptions include amine derivatives for which 1,2-addition results in an insoluble product (hydroxylamine, semicarbazide or the hydrazines).

Hydrogen cyanide also undergoes conjugate addition. A conjugated aldehyde or ketone may react with cyanide in the presence of acid. The reaction proceeds through a 1,4-addition pathway. Protonation of the oxygen Nucleophilic  attack Enol-keto tautomerization

1,2- and 1,4-Additions of Organometallic Reagents Organometallic reagents may attack the ,  -unsaturated carbonyl function in either 1,2- or 1,4-fashion. Organolithium reagents react almost exclusively by attacking the carbonyl carbon.

Grignard reagents with ,  -unsaturated aldehydes and ketones may give 1,2- addition, 1,4-addition or both, depending upon the particular substrates and conditions. Organocuprates are much more specific, undergoing primarily 1,4-addition reactions.

The first isolable intermediate in a copper-mediated 1,4-addition reaction is an enolate ion. This is trapped by the alkylating species.

Conjugate Additions of Enolate Ions: Michael Addition and Robinson Annulation Enolate ions undergo conjugate additions to ,  -unsaturated aldehydes and ketones in a reaction called the Michael addition.

With some Michael acceptors, the products of the initial addition are capable of a second intramolecular aldol condensation reaction, resulting in ring formation. This sequence of Michael addition followed by intramolecular aldol condensation is called a Robinson annulation.