Editing Ketene

{{short description|1=Organic compound of the form >C=C=O}}
{{About|the generic structure class|the specific chemical of this name|ethenone}}
[[File:Ketenes nonsymmetric.png|thumb|General formula for a ketene]]

In [[organic chemistry]], a '''ketene''' is an [[organic compound]] of the form {{chem2|RR'C\dC\dO}}, where R and R' are two arbitrary [[valence (chemistry)|monovalent]] [[functional group|chemical groups]] (or two separate [[Substituent|substitution sites]] in the same molecule).<ref name="Ullmann"/> The name may also refer to the specific compound [[ethenone]] {{chem2|H2C\dC\dO}}, the simplest ketene.<ref>{{cite book|title=Ketenes, Allenes and Related Compounds: Part 1, Volume 1|year=1980|editor=Saul Patai|isbn= 9780470771600|doi=10.1002/9780470771600|publisher=John Wiley & Sons|series=PATAI'S Chemistry of Functional Groups}}{{cite book|title=Ketenes, Allenes and Related Compounds: Part 2, Volume 2|year=1980|editor=Saul Patai|isbn= 9780471276708|doi=10.1002/9780470771617|publisher=John Wiley & Sons|series=PATAI'S Chemistry of Functional Groups}}</ref>

Although they are highly useful, most ketenes are [[chemical stability|unstable]].  When used as [[reagent]]s in a chemical procedure, they are typically generated when needed, and consumed as soon as (or while) they are produced.<ref name="Ullmann">{{Cite book |title=Ullmann's Encyclopedia of Industrial Chemistry |last1=Miller |first1=Raimund |last2=Abaecherli |first2=Claudio |last3=Said |first3=Adel |last4=Jackson |first4=Barry | name-list-style = vanc |year=2001 |isbn=978-3527306732 |chapter=Ketenes |doi=10.1002/14356007.a15_063}}</ref>

==History==
Ketenes were first studied as a class by [[Hermann Staudinger]] before 1905.<ref>{{Cite journal | first = Hermann | last = Staudinger | name-list-style = vanc |year=1905 |title=Ketene, eine neue Körperklasse |trans-title=Ketenes, a new class of substances |url=http://babel.hathitrust.org/cgi/pt?id=uc1.b3481909;view=1up;seq=483 |journal=Berichte der Deutschen Chemischen Gesellschaft |volume=38 |issue=2 |pages=1735–1739 |doi=10.1002/cber.19050380283}}</ref>

Ketenes were systematically investigated by Hermann Staudinger in 1905 in the form of diphenylketene (conversion of <math>\alpha</math>-chlorodiphenyl acetyl chloride with zinc). Staudinger was inspired by the first examples of reactive organic intermediates and stable radicals discovered by [[Moses Gomberg]] in 1900 (compounds with triphenylmethyl group).<ref>Thomas T. Tidwell, The first century of Ketenes (1905-2005): the birth of a family of reactive intermediates, Angewandte Chemie, Int. Edition, Band 44, 2005, S. 5778–5785</ref>

==Properties==
Ketenes are highly electrophilic at the carbon atom bonded with the heteroatom, due to its ''sp'' character. Ketenes can be formed with different heteroatoms bonded to the ''sp'' carbon atom, such as [[oxygen|O]], [[sulphur|S]] or [[selenium|Se]], respectively called ketenes, [[thioketene|thioketenes]] and selenoketenes.

[[Ethenone]], the simplest ketene, has different experimental lengths for each of its double bonds; the C=O bond is 1.160 [[Angstrom|Å]] and the C=C bond is 1.314 Å. The angle between the two [[Hydrogen|H]] atoms is 121.5°, similar to the theoretically ideal angle in [[alkene]]s between ''sp<sup>2</sup> ''carbon atoms and H substituents.<ref>{{Cite journal | first1 = Ngai Ling | last1 = Ma |first2 = MingWah | last2 = Wong | name-list-style = vanc |year=2000 |title=A Theoretical Study of the Properties and Reactivities of Ketene, Thioketene, and Selenoketene|url=https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/(SICI)1099-0690(200004)2000:8%3C1411::AID-EJOC1411%3E3.0.CO;2-N |journal=European Journal of Organic Chemistry |volume=2000 |issue=8 |pages=1411_1421 |doi=10.1002/(SICI)1099-0690(200004)2000:8<1411::AID-EJOC1411>3.0.CO;2-N}}</ref>

Ketenes are unstable and cannot be stored. In the absence of nucleophiles with which to react, ethenone dimerises to give β-[[lactone]], a cyclic [[ester]]. If the ketene is disubstituted, the dimerisation product is a substituted cyclobutadione. For monosubstituted ketenes, the dimerisation could afford either the ester or the diketone product.

==Synthesis==
Ethenone is produced on a commercial scale by thermal dehydration of [[acetic acid]]. Substituted ketenes can be prepared from [[acyl chloride]]s by an [[elimination reaction]] in which [[hydrogen chloride|HCl]] is lost:

:[[File:Mecanisme-de-la-formation-des-cetenes.png|none|Formation of a ketene from an acyl chloride.]]
In this reaction, a base, usually [[triethylamine]], removes the [[acid]]ic [[proton]] alpha to the [[carbonyl]] group, inducing the formation of the carbon-carbon double bond and the loss of a [[chloride]] ion:

[[File:Ketene Synthesis.png|370px|Synthesis of Ketene]]

Ketenes can also be formed from α-[[diazoketone]]s by the [[Wolff rearrangement]], and from [[vinylene carbonate]] by [[phosphorus(V) sulfide]] and irradiation.<ref>Handbook of Reagents for Organic Syntheses, ''Sulfur-Containing Reagents'', ed. L.A. Paquette, Wiley-VCH, 2010, {{ISBN|978-0-470-74872-5}}, p.&nbsp;535.</ref>

Another way to generate ketenes is through [[flash vacuum thermolysis]] (FVT) with 2-[[pyridylamine]]s. Plüg and Wentrup developed a method in 1997 that improved on FVT reactions to produce ketenes with a stable FVT that is moisture insensitive, using mild conditions (480&nbsp;°C). The N-pyridylamines are prepared via a condensation with R-[[malonate]]s with N-amino([[pyridene]]) and [[N,N'-Dicyclohexylcarbodiimide|DCC]] as the solvent.<ref>{{Cite journal |last=Carsten Plüg ,Hussein Kanaani and Curt Wentrup |date=12 February 2015 |title=Ketenes from N-(2-Pyridyl)amides |journal=Australian Journal of Chemistry |volume=68 |issue=4 |pages=687 |doi=10.1071/CH14714 }}</ref>

A more robust method for preparing ketenes is the [[carbonylation]] of [[Transition metal carbene complex|metal-carbenes]], and ''in situ'' reaction of the thus produced highly reactive ketenes with suitable reagents such as [[imines]], [[amines]], or [[alcohols]].<ref>{{cite journal | vauthors = Paul ND, Chirila A, Lu H, Zhang XP, de Bruin B | title = Carbene radicals in cobalt(II)-porphyrin-catalysed carbene carbonylation reactions; a catalytic approach to ketenes | journal = Chemistry: A European Journal | volume = 19 | issue = 39 | pages = 12953–8 | date = September 2013 | pmid = 24038393 | pmc = 4351769 | doi = 10.1002/chem.201301731}}</ref> This method is an efficient [[One-pot synthesis|one‐pot]] tandem protocol of the carbonylation of α‐diazocarbonyl compounds and a variety of ''N''‐tosylhydrazones catalysed by Co(II)–[[porphyrin]] metalloradicals leading to the formation of ketenes, which subsequently react with a variety of [[nucleophile]]s and imines to form [[esters]], [[amides]] and [[Beta-lactam|β‐lactams]]. This system has a broad substrate scope and can be applied to various combinations of [[carbene]] precursors, nucleophiles and imines.<ref>{{Cite journal| vauthors = Chirila A, van Vliet KM, Paul ND, de Bruin B |date=2018|title=[Co(MeTAA)] Metalloradical Catalytic Route to Ketenes via Carbonylation of Carbene Radicals|journal=European Journal of Inorganic Chemistry|language=en|volume=2018|issue=20–21|pages=2251–2258|doi=10.1002/ejic.201800101|issn=1099-0682|url=https://pure.uva.nl/ws/files/34388611/Chirila_et_al_2018_European_Journal_of_Inorganic_Chemistry.pdf|doi-access=free}}</ref>

[[Ethenone]] can be produced through [[pyrolysis]] of [[acetone]] vapours over a hot filament in an apparatus that was eventually developed into the "ketene lamp" or "Hurd lamp" (named for Charles D. Hurd).<ref>{{Cite journal |last=Tidwell |first=Thomas T. |date=2005-09-12 |title=The First Century of Ketenes (1905–2005): The Birth of a Versatile Family of Reactive Intermediates |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.200500098 |journal=Angewandte Chemie International Edition |language=en |volume=44 |issue=36 |pages=5778–5785 |doi=10.1002/anie.200500098 |issn=1433-7851}}</ref>

==Reactions and applications==
Due to their [[cumulated double bond]]s, ketenes are very reactive.<ref name=":1">{{citation|surname1=Siegfried Hauptmann|title=Organische Chemie: mit 65 Tabellen|publisher=Deutscher Verlag für Grundstoffindustrie|location=Leipzig|at=pp.&nbsp;410–412|isbn=3871449024|date=1985|language=de
}}</ref>

=== Formation of carboxylic acid esters ===
By reaction with alcohols, [[carboxylic acid ester]]s are formed:

:[[File:Ketene_Reaktion1_V1.svg|380x380px|none]]

=== Formation of carboxylic anhydrides ===
Ketenes react with a carboxylic acids to form [[carboxylic acid anhydride]]s:

:[[File:Ketene_Reaktion2_V1.svg|450x450px|none]]

=== Formation of amides ===
Ketenes react with [[ammonia]] and [[amine]]s to give the corresponding [[amide]]s:
:[[File: Ketene Reaktion4 V1.svg|frameless|none|380x380px]]

=== Hydrolysis ===
By reaction with water, carboxylic acids are formed from ketenes:

:[[File: Ketene Reaktion6 V1.svg|frameless|none|350x350px]]

=== Formation of enol esters ===
Enol esters are formed from ketenes with [[enol]]isable [[carbonyl compounds]]. The following example shows the reaction of [[ethenone]] with [[acetone]] to form a propen-2-yl acetate:

:[[File: Ketene Reaktion7 V3 unlabeled.svg|frameless|none|480x480px]]

=== Dimerisation ===
At room temperature, ketene quickly dimerizes to [[diketene]], but the ketene can be recovered by heating:

:[[File:Dimerisation of ketene.png|frameless|Dimerisation of ketene|500px]]

=== [2+2]-cycloaddition ===
Ketenes can react with [[alkenes]], carbonyl compounds, [[carbodiimides]] and imines in a [[Enone–alkene cycloadditions|[2+2] cycloaddition]]. The example shows the synthesis of a [[β-lactam]] by the reaction of a ketene with an imine (see [[Staudinger synthesis]]):<ref name=":2">{{citation|surname1=Jie Jack Li|title=Name reactions. A collection of detailed reaction mechanisms|edition=3|publisher=Springer-Verlag|location=Berlin|at=pp.&nbsp;561-562|isbn=9783540300304|date=2006|language=de|doi=10.1007/3-540-30031-7
}}</ref><ref name=":3">{{citation|surname1=Hermann Staudinger|periodical=[[Justus Liebigs Annalen der Chemie]]|title=Zur Kenntnis der Ketene. Diphenylketen|volume=356|issue=1–2|publisher=John Wiley & Sons, Inc.|at=pp.&nbsp;51–123|date=1907|language=de|doi=10.1002/jlac.19073560106
|url=https://zenodo.org/record/1427571}}</ref>
:[[File:Staudinger-Synthese ÜV6.svg|frameless|none|450x450px]]

===Applications===
Ketenes are generally very reactive, and participate in various [[ketene cycloaddition]]s. One important process is the dimerization to give [[beta-propiolactone|propiolactone]]s. A specific example is the dimerization of the ketene of [[stearic acid]] to afford [[alkyl ketene dimer]]s, which are widely used in the paper industry.<ref name="Ullmann" /> AKD's react with the hydroxyl groups on the cellulose via [[esterification]] reaction.

They will also undergo [2+2] cycloaddition reactions with electron-rich [[alkyne]]s to form [[cyclobutenone]]s, or carbonyl groups to form beta-[[lactone]]s. With [[imine]]s, beta-lactams are formed. This is the Staudinger synthesis, a facile route to this important class of compounds. With [[acetone]], ketene reacts to give [[isopropenyl acetate]].<ref name="Ullmann" />

A variety of [[hydroxyl group|hydroxylic]] compounds can add as nucleophiles, forming either [[enol]] or [[ester]] products. As examples, a water molecule easily adds to ketene to give [[1,1-dihydroxyethene]] and [[acetic anhydride]] is produced by the reaction of [[acetic acid]] with ketene. Reactions between [[diol]]s ({{chem2|HO\sR\sOH}}) and bis-ketenes ({{chem2|O\dC\dCH\sR'\sCH\dC\dO}}) yield [[polyester]]s with a repeat unit of ({{chem2|\sO\sR\sO\sCO\sR'\sCO}}).

[[Ethyl acetoacetate]], an important starting material in organic synthesis, can be prepared using a [[diketene]] in reaction with [[ethanol]]. They directly form ethyl acetoacetate, and the yield is high when carried out under controlled circumstances; this method is therefore used industrially.

== See also ==
*[[Ynol]]
*[[Thioketene]]

== References ==
{{Reflist}}

== External links ==
*{{Commons category-inline}}

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[[Category:Ketenes| ]]
[[Category:Functional groups]]