Ketene
A ketene is an organic compound of the form R′R″C=C=O. Ketene also refers to CH2=C=O, occasionally called ethenone. Although they are highly useful, ketene and its various derivatives are rarely isolated.
Ketenes were first studied as a class by Hermann Staudinger.
Ketene, the simplest ketene, can be formed by pyrolysis (thermal cracking) of acetone:
This reaction is called the Schmidlin ketene synthesis.
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.
Another way to generate ketenes is through flash vacuum thermolysis 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.
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. With acetone, ketene reacts to give Isopropenyl acetate.
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