Ketene
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In organic chemistry, a ketene is an organic compound of the form Template:Chem2, where R and R' are two arbitrary monovalent chemical groups (or two separate substitution sites in the same molecule).<ref name="Ullmann"/> The name may also refer to the specific compound ethenone Template:Chem2, the simplest ketene.<ref>Template:Cite bookTemplate:Cite book</ref>
Although they are highly useful, most ketenes are unstable. When used as reagents in a chemical procedure, they are typically generated when needed, and consumed as soon as (or while) they are produced.<ref name="Ullmann">Template:Cite book</ref>
HistoryEdit
Ketenes were first studied as a class by Hermann Staudinger before 1905.<ref>Template:Cite journal</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>
PropertiesEdit
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 O, S or Se, respectively called ketenes, thioketenes and selenoketenes.
Ethenone, the simplest ketene, has different experimental lengths for each of its double bonds; the C=O bond is 1.160 Å and the C=C bond is 1.314 Å. The angle between the two H atoms is 121.5°, similar to the theoretically ideal angle in alkenes between sp2 carbon atoms and H substituents.<ref>Template:Cite journal</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.
SynthesisEdit
Ethenone is produced on a commercial scale by thermal dehydration of acetic acid. Substituted ketenes can be prepared from acyl chlorides by an elimination reaction in which HCl is lost:
In this reaction, a base, usually triethylamine, removes the acidic proton alpha to the carbonyl group, inducing the formation of the carbon-carbon double bond and the loss of a chloride ion:
Ketenes can also be formed from α-diazoketones 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, Template:ISBN, p. 535.</ref>
Another way to generate ketenes is through flash vacuum thermolysis (FVT) with 2-pyridylamines. 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 °C). The N-pyridylamines are prepared via a condensation with R-malonates with N-amino(pyridene) and DCC as the solvent.<ref>Template:Cite journal</ref>
A more robust method for preparing ketenes is the carbonylation of metal-carbenes, and in situ reaction of the thus produced highly reactive ketenes with suitable reagents such as imines, amines, or alcohols.<ref>Template:Cite journal</ref> This method is an efficient 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 nucleophiles and imines to form esters, amides and β‐lactams. This system has a broad substrate scope and can be applied to various combinations of carbene precursors, nucleophiles and imines.<ref>Template:Cite journal</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>Template:Cite journal</ref>
Reactions and applicationsEdit
Due to their cumulated double bonds, ketenes are very reactive.<ref name=":1">Template:Citation</ref>
Formation of carboxylic acid estersEdit
By reaction with alcohols, carboxylic acid esters are formed:
Formation of carboxylic anhydridesEdit
Ketenes react with a carboxylic acids to form carboxylic acid anhydrides:
Formation of amidesEdit
Ketenes react with ammonia and amines to give the corresponding amides:
HydrolysisEdit
By reaction with water, carboxylic acids are formed from ketenes:
Formation of enol estersEdit
Enol esters are formed from ketenes with enolisable carbonyl compounds. The following example shows the reaction of ethenone with acetone to form a propen-2-yl acetate:
DimerisationEdit
At room temperature, ketene quickly dimerizes to diketene, but the ketene can be recovered by heating:
[2+2]-cycloadditionEdit
Ketenes can react with alkenes, carbonyl compounds, carbodiimides and imines in a [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">Template:Citation</ref><ref name=":3">Template:Citation</ref>
ApplicationsEdit
Ketenes are generally very reactive, and participate in various ketene cycloadditions. One important process is the dimerization to give propiolactones. A specific example is the dimerization of the ketene of stearic acid to afford alkyl ketene dimers, 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 alkynes to form cyclobutenones, or carbonyl groups to form beta-lactones. With imines, 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 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 diols (Template:Chem2) and bis-ketenes (Template:Chem2) yield polyesters with a repeat unit of (Template:Chem2).
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 alsoEdit
ReferencesEdit
External linksEdit
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