A ketene is an organic compound of the form R'R''C=C=O. The term is also used specifically to mean ethenone, the simplest ketene, where R' and R'' are hydrogen atoms.
Ketenes were first studied as a class by Hermann Staudinger.
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 Wolff rearrangement.
Phenylacetic acid in the presence of a base will lose water to produce phenylketene due to the high acidity of the alpha proton.
Ketenes can be formed by pyrolysis (thermal cracking) of acetone:
This reaction is called the Schmidlin ketene synthesis.
Ketenes are generally very reactive, and participate in various cycloadditions. 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.
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, a very 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 good, so this method is used industrially.
Full article ▸