Ketene

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ketene

[′kē‚tēn]
(organic chemistry)
C2H2O A colorless, toxic, highly reactive gas, with disagreeable taste; boils at -56°C; soluble in ether and acetone, and decomposes in water and alcohol; used as an acetylating agent in organic synthesis.

Ketene

 

carbomethene, the first member of the ketene series (RR’C=C=0), the unsaturated, highly reactive organic compound CH2=C=0.

Ketene is a gas (boiling point, 41°C; melting point, 134.6°C) and serves as an effective acetylizing agent; acetylation products form upon the reaction of ketene with compounds containing a mobile hydrogen atom (for example, alcohols, thiols, amines, and acids):

ROH + CH2=C= O → ROCOCH3

Ketene combines readily with water to form acetic acid and ketones. For example, dimethyl-β-propiolactone is obtained from ketene and acetone:

Ketene is stable at –80°C. At 0°C it readily undergoes dimerization to form diketene (methylene-β-propiolactone)

from which ketene can be regenerated by pyrolysis at 550°-600°C. Ketene is similar to phosgene in terms of toxicity.

The industrial preparation of ketene involves the pyrolysis of acetic acid in the presence of triethyl phosphate or the pyrolysis of acetone over alumina. The common method of synthesizing ketene and other compounds of the ketene series consists in the separation of halogens from the acid halides of α-halocarboxylic acids:

RR’CX—COX→ RR’C = C = 0 + ZnX2

(X being a halogen atom)

Ketene is easily obtained in the laboratory by the pyrolysis of acetone or acetic anhydride in a special apparatus called a ketene lamp.

Ketene is used in industry for converting acetic acid to anhydride, preparing acetylcellulose, and synthesizing propiolactone.

REFERENCES

Lacey, R. N. “Keten ν organicheskom sinteze.” In the book Uspekhi organicheskoi khimii, vol. 2. Moscow, 1964. Page 204. (Translated from English.)

IA. F. KOMISSAROV