A ketene is an organic compound of the form R′R″C=C=O. Ketene also refers to the specific compound CH2=C=O, the simplest ketene molecule, which is occasionally called ethenone. Although they are highly useful, most ketenes are unstable, so they are not usually isolated during a reaction process.
- CH3−CO−CH3 → CH2=C=O + CH4
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.
Reactions and applicationsEdit
Ketenes are generally very reactive, and participate in various 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. AKD's react with the hydroxyl groups on the celluose 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.
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 (HO−R−OH) and bis-ketenes (O=C=CH−R′−CH=C=O) yield polyesters with a repeat unit of (−O−R−O−CO−R′−CO).
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.
- Miller, Raimund; Abaecherli, Claudio; Said, Adel; Jackson, Barry (2001). "Ketenes". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a15_063. ISBN 978-3527306732.
- Hermann Staudinger (1905). "Ketene, eine neue Körperklasse" [Ketenes, a new class of substances]. Berichte der Deutschen Chemischen Gesellschaft. 38 (2): 1735–1739. doi:10.1002/cber.19050380283.
- Weygand, Conrad (1972). Hilgetag, G.; Martini, A., eds. Weygand/Hilgetag Preparative Organic Chemistry (4th ed.). New York: John Wiley & Sons, Inc. pp. 1031–1032. ISBN 978-0471937494.
- Ketene in Organic Syntheses Organic Syntheses, Submitted by C. D. Hurd Checked by Oliver Kamm Coll. Vol. 1, p.330 (1941); Vol. 4, p.39 (1925). Link
- Julius Schmidlin and Maximilian Bergman (1910) "Darstellung des Ketens aus Aceton" (Preparation of ketene from acetone), Berichte der deutschen chemischen Gesellschaft, 43 (3) : 2821-2823.
- Carsten Plüg ,Hussein Kanaani and Curt Wentrup (12 February 2015). "Ketenes from N-(2-Pyridyl)amides". Australian Journal of Chemistry. 68 (4): 687. doi:10.1071/CH14714. Retrieved 21 February 2018.