Schmidlin ketene synthesis
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.
Ketenes were first studied as a class by Hermann Staudinger.
Ketene, the simplest ketene, is produced on a large scale industrially for use in the production of acetic anhydride. It can also be generated 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 (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.
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