View source for Chemistry:Α,β-Unsaturated carbonyl compound

{{Short description|Functional group of organic compounds}}
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[[File:α,β-unsaturated labeled.svg|thumb|General structure of α,β-unsaturated carbonyl compounds.{{Br}}R<sub>2</sub> & R<sub>4</sub> can also be single hydrogens.|228x228px]]
'''α,β-Unsaturated carbonyl compounds''' are [[Chemistry:Organic compound|organic compound]]s with the general structure (O=CR)−C<sup>α</sup>=C<sup>β</sup>-R.<ref>{{cite book |doi=10.1002/9780470772218 |title=Enones: Vol. 1 (1989) |date=1989 |isbn=9780470772218 |editor-last1=Patai |editor-last2=Rappoport |editor-first1=Saul |editor-first2=Zvi|series=Patai's Chemistry of Functional Groups }}</ref><ref>{{cite book |doi=10.1002/9780470772225 |title=Enones: Vol. 2 (1989) |date=1989 |isbn=9780470772225 |editor-last1=Patai |editor-last2=Rappoport |editor-first1=Saul |editor-first2=Zvi|series=Patai's Chemistry of Functional Groups }}</ref> Such compounds include [[Chemistry:Enone|enone]]s and enals, but also carboxylic acids and the corresponding esters and amides. In these compounds the [[Chemistry:Carbonyl group|carbonyl group]] is [[Chemistry:Conjugated system|conjugated]] with an [[Chemistry:Alkene|alkene]] (hence the adjective [[Chemistry:Saturated and unsaturated compounds|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 [[Chemistry:Acrolein|acrolein]] (propenal), [[Chemistry:Mesityl oxide|mesityl oxide]], [[Chemistry:Acrylic acid|acrylic acid]], and [[Chemistry:Maleic acid|maleic acid]]. Unsaturated carbonyls can be prepared in the laboratory in an [[Chemistry:Aldol reaction|aldol reaction]] and in the [[Chemistry:Perkin reaction|Perkin reaction]].

{{Br}}

==Classifications==
α,β-Unsaturated carbonyl compounds can be subclassified according to the nature of the carbonyl and alkene groups.
<gallery caption="Parent α,β-Unsaturated Carbonyls" widths="120px" heights="100px" >
File:MVK-2D-skeletal.png|[[Chemistry:Methyl vinyl ketone|Methyl vinyl ketone]], the simplest α,β-unsaturated ketone
File:Acrolein-skeletal.png|[[Chemistry:Acrolein|Acrolein]], the simplest α,β-unsaturated aldehyde
File:Methyl acrylate.png|[[Chemistry:Methyl acrylate|Methyl acrylate]], an α,β-unsaturated ester
File:Acrylamide-2D-skeletal.png|[[Chemistry:Acrylamide|Acrylamide]], precursor to polyacrylamide
File:Maleinsäure.svg|[[Chemistry:Maleic acid|Maleic acid]], an α,β-unsaturated ''di''carbonyl
File:Fumaric-acid-2D-skeletal.png|[[Chemistry:Fumaric acid|Fumaric acid]], isomeric with maleic acid
File:Acryloyl chloride2.png|[[Chemistry:Acryloyl chloride|Acryloyl chloride]]
File:Chalcone.svg|[[Chemistry:Chalcone|Chalcone]]
</gallery>

===Acryloyl group===
[[Image:Acrylgruppe.svg|thumb|200px|right|Structure of the acryloyl group]]
α,β-Unsaturated carbonyl compounds featuring a carbonyl conjugated to an alkene that is terminal, or vinylic, contain the '''acryloyl group''' ([[Chemistry:Hydrogen|H]]<sub>2</sub>[[Chemistry:Carbon|C]]=CH−C(=[[Chemistry:Oxygen|O]])−); it is the acyl group derived from [[Chemistry:Acrylic acid|acrylic acid]]. The [[Chemistry:Preferred IUPAC name|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".

===α,β-Unsaturated acids, esters, and amides===
An '''α,β-unsaturated acid''' is a type of α,β-unsaturated carbonyl compound that consists of an [[Chemistry:Alkene|alkene]] [[Chemistry:Conjugated system|conjugated]] to a [[Chemistry:Carboxylic acid|carboxylic acid]].<ref name=March>{{March6th}}</ref> The simplest example is acrylic acid (CH<sub>2</sub>=CHCO<sub>2</sub>H). These compounds are prone to polymerization, giving rise to the large area of polyacrylate plastics. [[Physics:Acrylate polymer|Acrylate polymer]]s are derived from but do not contain the acrylate group.<ref name=Ullmann>{{Ullmann|doi= 10.1002/14356007.a01_161.pub2|title= Acrylic Acid and Derivatives|year= 2003|last1= Ohara|first1= Takashi|last2= Sato|first2= Takahisa|last3= Shimizu|first3= Noboru|last4= Prescher|first4= Günter|last5= Schwind|first5= Helmut|last6= Weiberg|first6= Otto|last7= Marten|first7= Klaus|last8= Greim|first8= Helmut|isbn= 3527306730}}</ref> The carboxyl group of acrylic acid can react with [[Chemistry:Ammonia|ammonia]] to form [[Chemistry:Acrylamide|acrylamide]], or with an [[Chemistry:Alcohol|alcohol]] to form an acrylate [[Chemistry:Ester|ester]]. Acrylamide and [[Chemistry:Methyl acrylate|methyl acrylate]] are commercially important examples of α,β-unsaturated [[Chemistry:Amide|amide]]s and α,β-unsaturated esters, respectively. They also polymerize readily. Acrylic acid, its esters, and its amide derivatives feature the acryloyl group. 

α,β-Unsaturated ''di''carbonyls are also common. The parent compounds are [[Chemistry:Maleic acid|maleic acid]] and the isomeric [[Chemistry:Fumaric acid|fumaric acid]]. Maleic acid forms esters, an imide, and an anhydride, i.e. [[Chemistry:Diethyl maleate|diethyl maleate]], [[Chemistry:Maleimide|maleimide]], and [[Chemistry:Maleic anhydride|maleic anhydride]].  Fumaric acid, as fumarate, is an intermediate in the Krebs citric acid cycle, which is of great importance in bioenergy.

===Enones===
An '''enone''' (or '''alkenone''') is an organic compound containing both [[Chemistry:Alkene|alkene]] and [[Chemistry:Ketone|ketone]] functional groups. In an α,β-unsaturated enone, the alkene is [[Chemistry:Conjugated system|conjugated]] to the carbonyl group of the ketone.<ref name=March/> The simplest enone is [[Chemistry:Methyl vinyl ketone|methyl vinyl ketone]] (butenone, CH<sub>2</sub>=CHCOCH<sub>3</sub>). Enones are typically produced using an [[Chemistry:Aldol condensation|aldol condensation]] or [[Chemistry:Knoevenagel condensation|Knoevenagel condensation]]. Some commercially significant enones produced by condensations of [[Chemistry:Acetone|acetone]] are [[Chemistry:Mesityl oxide|mesityl oxide]] ([[Chemistry:Dimer|dimer]] of acetone) and [[Chemistry:Phorone|phorone]] and [[Chemistry:Isophorone|isophorone]] (trimers).<ref>{{Ullmann |doi=10.1002/14356007.a15_077|title=Ketones|year=2000|last1=Siegel|first1=Hardo|last2=Eggersdorfer|first2=Manfred|isbn=9783527306732}}</ref>  In the [[Chemistry:Meyer–Schuster rearrangement|Meyer–Schuster rearrangement]], the starting compound is a [[Chemistry:Propargyl|propargyl]] [[Chemistry:Alcohol|alcohol]]. Another method to access α,β-unsaturated carbonyls is via [[Chemistry:Selenoxide elimination|selenoxide elimination]]. Cyclic enones can be prepared via the [[Chemistry:Pauson–Khand reaction|Pauson–Khand reaction]].

[[File:Condensationaldolique.png|600px|center|thumb|General reaction for an aldol condensation between two carbonyl compounds]]

====Cyclic enones====
The cyclic enones include [[Chemistry:Cyclopropenone|cyclopropenone]], cyclobutenone,<ref>{{cite journal |doi=10.15227/orgsyn.089.0491 |title=Preparation of Cyclobutenone |journal=Organic Syntheses |date=2012 |volume=89 |page=491|author=A. G. Ross, X. Li, S. J. Danishefsky |doi-access=free }}</ref> [[Chemistry:Cyclopentenone|cyclopentenone]], [[Chemistry:Cyclohexenone|cyclohexenone]], and cycloheptenone.<ref>{{cite journal |doi=10.15227/orgsyn.059.0113 |title=One-Carbon Ring Expansion of Cycloalkanones to Conjugated Cycloalkenones: 2-Cyclohepten-1-One |journal=Organic Syntheses |date=1979 |volume=59 |page=113|author=Y. Ito, S. Fujii, M. Nakatuska, F. Kawamoto,T. Saegusa }}</ref>

===Enals===
An '''enal''' (or '''alkenal''') is an organic compound containing both [[Chemistry:Alkene|alkene]] and [[Chemistry:Aldehyde|aldehyde]] functional groups. In an α,β-unsaturated enal, the alkene is [[Chemistry:Conjugated system|conjugated]] to the carbonyl group of the aldehyde (formyl group).<ref name=March/> The simplest enal is [[Chemistry:Acrolein|acrolein]] (CH<sub>2</sub>=CHCHO). Other examples include [[Chemistry:Cis-3-Hexenal|''cis''-3-hexenal]] (essence of mowed lawns) and [[Chemistry:Cinnamaldehyde|cinnamaldehyde]] (essence of cinnamon).

<gallery caption="Other α,β-Unsaturated Carbonyls" widths="120px" heights="100px" >
File:Crotonaldehyde.png|''E''-[[Chemistry:Crotonaldehyde|Crotonaldehyde]], an enal that exists as an isomer
File:Cyclohexenone.png|[[Chemistry:Cyclohexenone|Cyclohexenone]], common cyclic enone
File:Testosteron.svg|[[Chemistry:Testosterone|testosterone]], the male sex hormone
File:Zimtaldehyd - cinnamaldehyde.svg|[[Chemistry:Cinnamaldehyde|Cinnamaldehyde]], essence of cinnamon
File:P-Benzochinon.svg|Paraquinone, a particularly electrophilic α,β-unsaturated carbonyl
File:(benzylideneacetone)iron-tricarbonyl-2D-skeletal.png|Enone complex of iron tricarbonyl
File:Squaric acid.svg|[[Chemistry:Squaric acid|Squaric acid]]
</gallery>

==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 [[Chemistry:Nucleophile|nucleophile]]s. 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 [[Chemistry:Mesityl oxide|mesityl oxide]] into [[Chemistry:Isophorone|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 [[Chemistry:Diels–Alder reaction|Diels–Alder reaction]]s.  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 [[Chemistry:(Benzylideneacetone)iron tricarbonyl|Fe(bda)(CO)<sub>3</sub>]] and [[Chemistry:Tris(dibenzylideneacetone)dipalladium(0)|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 [[Chemistry:Nazarov cyclization reaction|Nazarov cyclization reaction]] and in the [[Chemistry:Rauhut–Currier reaction|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 [[Biology:Acyl-CoA dehydrogenase|acyl-CoA dehydrogenase]]s.<ref>{{cite journal | vauthors = Thorpe C, Kim JJ | title = Structure and mechanism of action of the acyl-CoA dehydrogenases | journal = FASEB Journal | volume = 9 | issue = 9 | pages = 718–25 | date = June 1995 | pmid = 7601336 | doi = 10.1096/fasebj.9.9.7601336 | s2cid = 42549744}}</ref> [[Chemistry:Flavin adenine dinucleotide|Flavin adenine dinucleotide]] (FAD) is a required co-factor.

[[Image:Beta-Oxidation1.svg|500px|center]]

==Safety==
Since α,β-unsaturated compounds are electrophiles and alkylating agents, many α,β-unsaturated carbonyl compounds are toxic. The endogenous scavenger compound [[Chemistry:Glutathione|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 ==
* [[Biology:2-alkenal reductase|2-alkenal reductase]]
* [[Biology:Enol|Enol]]

==References==
{{Reflist}}


[[Category:Functional groups]]
[[Category:Conjugated ketones]]
[[Category:Alkene derivatives]]
[[Category:Enones| ]]

{{Sourceattribution|Α,β-Unsaturated carbonyl compound}}