The simplest enal is acrolein (CH2=CHCHO). Other examples include cis-3-hexenal (essence of mowed lawns) and cinnamaldehyde (essence of cinnamon). Reactions of α,β-unsaturated carbonyls 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 are 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. Enones undergo the Nazarov cyclization reaction and in the Rauhut–Currier reaction (dimerization). α,β-Unsaturated thioesters α,β-Unsaturated thioesters are intermediates in several enzymatic processes. Two prominent examples are coumaroyl-coenzyme A and crotonyl-coenzyme A. They arise by the action of acyl-CoA dehydrogenases. Flavin adenine dinucleotide (FAD) is a required co-factor. Safety 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. See also 2-alkenal reductase Enol References Functional groups Conjugated ketones Alkene derivatives