α,β-Unsaturated carbonyl compounds are organic compounds with the general structure (O=CR)−Cα=Cβ-R.[1][2] Such compounds include enones and enals, but also carboxylic acids and the corresponding esters and amides. In these compounds, the carbonyl group is conjugated with an alkene (hence the adjective unsaturated). Unlike the case for carbonyls without a flanking alkene group, α,β-unsaturated carbonyl compounds are susceptible to attack by nucleophiles at the β-carbon. This pattern of reactivity is called vinylogous. Examples of unsaturated carbonyls are acrolein (propenal), mesityl oxide, acrylic acid, and maleic acid. Unsaturated carbonyls can be prepared in the laboratory in an aldol reaction and in the Perkin reaction.
General structure of α,β-unsaturated carbonyl compounds. R2 & R4 can also be single hydrogens.
α,β-Unsaturated carbonyl compounds featuring a carbonyl conjugated to an alkene that is terminal, or vinylic, contain the acryloyl group (H2C=CH−C(=O)−); it is the acyl group derived from acrylic acid. The preferred IUPAC name for the group is prop-2-enoyl, and it is also known as acrylyl or simply (and incorrectly) as acryl. Compounds containing an acryloyl group can be referred to as "acrylic compounds".
An α,β-unsaturated acid is a type of α,β-unsaturated carbonyl compound that consists of an alkeneconjugated to a carboxylic acid.[3] The simplest example is acrylic acid (CH2=CHCO2H). These compounds are prone to polymerization, giving rise to the large area of polyacrylate plastics. Acrylate polymers are derived from but do not contain the acrylate group.[4] The carboxyl group of acrylic acid can react with ammonia to form acrylamide, or with an alcohol to form an acrylate ester. Acrylamide and methyl acrylate are commercially important examples of α,β-unsaturated amides and α,β-unsaturated esters, respectively. They also polymerize readily. Acrylic acid, its esters, and its amide derivatives feature the acryloyl group.
An enal (or alkenal) is an organic compound containing both alkene and aldehyde functional groups. In an α,β-unsaturated enal, the alkene is conjugated to the carbonyl group of the aldehyde (formyl group).[3] The simplest enal is acrolein (CH2=CHCHO). Other examples include cis-3-hexenal (essence of mowed lawns) and cinnamaldehyde (essence of cinnamon).
α,β-Unsaturated carbonyls are electrophilic at both the carbonyl carbon as well as the β-carbon. Depending on conditions, either site is attacked by nucleophiles. Additions to the alkene are called conjugate additions. One type of conjugate addition is the Michael addition, which is used commercially in the conversion of mesityl oxide into isophorone. Owing to their extended conjugation, α,β-unsaturated carbonyls are prone to polymerization. In terms of industrial scale, polymerization dominates the use of α,β-unsaturated carbonyls. Again because of their electrophilic character, the alkene portion of α,β-unsaturated carbonyls is good dienophiles in Diels–Alder reactions. They can be further activated by Lewis acids, which bind to the carbonyl oxygen. α,β-Unsaturated carbonyls are good ligands for low-valent metal complexes, examples being Fe(bda)(CO)3 and tris(dibenzylideneacetone)dipalladium(0).
α,β-Unsaturated carbonyls are readily hydrogenated. Hydrogenation can target the carbonyl or the alkene (conjugate reduction) selectively, or both functional groups.
Since α,β-unsaturated compounds are electrophiles and alkylating agents, many α,β-unsaturated carbonyl compounds are toxic. The endogenous scavenger compound glutathione naturally protects from toxic electrophiles in the body. Some drugs (amifostine, N-acetylcysteine) containing thiol groups may protect from such harmful alkylation.
^Ito, Y.; Fujii, S.; Nakatuska, M.; Kawamoto, F.; Saegusa, T. (1979). "One-Carbon Ring Expansion of Cycloalkanones to Conjugated Cycloalkenones: 2-Cyclohepten-1-One". Organic Syntheses. 59: 113. doi:10.15227/orgsyn.059.0113.
Acrylamide, precursor to polyacrylamide
Fumaric acid, isomeric with maleic acid
E-Crotonaldehyde, an enal that exists as an isomer
Cyclohexenone, common cyclic enone
testosterone, the male sex hormone
Cinnamaldehyde, essence of cinnamon
Enone complex of iron tricarbonyl
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