Ketene

[kee-teen]

A ketene is an organic compound of the form R₂C=C=O. Hermann Staudinger pioneered the research of ketenes. Ketene also refers to ethenone, the specific compound of this class in which both Rs are hydrogen.

Properties

Ethenone is colourless and has a sharp odour, and exposure to concentrated levels causes humans to experience irritation of body parts such as the eye, nose, throat, and lung. Pulmonary edema has also been noted if these levels are experienced for an extended period of time. It has a melting point of -150.5 °C (-239 °F) and a boiling point of -56.1 °C (-69 °F).

Formation

Ketenes can be prepared from acyl chlorides by an elimination reaction in which HCl is lost.

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 are formed in the Wolff rearrangement from α-diazoketones
Phenylacetic acid in the presence of base will lose water to produce phenylketene due to the high acidity of the alpha proton.
Ethenone (CH₂=C=O) can be prepared in the laboratory by pyrolysis of acetone vapor:

CH₃−CO−CH₃ + Δ → CH₂=C=O + CH₄

Reactions

Ketene is very reactive, tending to attach itself to other molecules in the form of an acetyl group. It will react with itself to form cyclic dimers known as diketenes (oxetanones). It will also undergo [2+2] cycloaddition reactions to electron-rich alkynes to form cyclobutenones.

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-).

Ethenone reacts with acetic acid to form acetic anhydride.