Ketene

This article is about the generic structure class. For the specific chemical of this name, see ethenone.

General formula for a ketene.

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.

Formation

Ethenone, the simplest ketene, can be formed by pyrolysis (thermal cracking) of acetone:

CH₃−CO−CH₃ + ΔT → CH₂=C=O + CH₄

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.

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 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 high when carried out under controlled circumstances; this method is therefore used industrially.

See also

• Ynolate
• Thioketene
• Ethenone