ketene

General formula for a ketene.

A ketene is an organic compound of the form R'R''C=C=O where R',R'' are arbitrary radicals. The term is also used specifically to mean ethenone, the simplest ketene, where R' and R'' are hydrogens.

Ketenes were first studied as a class by Hermann Staudinger.^[1]

Formation

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 chloride ion.

Ketenes can be formed also from α-diazoketones by Wolff rearrangement.

Phenylacetic acid in the presence of 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:

CH₃−CO−CH₃ + Δ → CH₂=C=O + CH₄

Reactions

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 -β-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 widely used in industry.

See also

• Ynolate

References