Ethenone is the formal name for ketene, an organic compound with formula C2H2O or H2C=C=O. It is the simplest member of the ketene class. It is a tautomer of the even less stable ethynol.
Properties
Ethenone is a highly reactive gas and has a sharp irritating odour. It is only reasonably stable at low temperatures. It must therefore always be prepared for each use and processed immediately, otherwise a dimerization to diketene occurs or it reacts to polymers that are difficult to handle. The polymer content formed during the preparation is reduced, for example, by adding sulfur dioxide to the ketene gas. Because of its cumulative double bonds, ethenone is highly reactive and reacts in an addition reaction H-acidic compounds to the corresponding acetic acid derivatives. It does for example react with water to acetic acid or with primary or secondary amines to the corresponding acetamides. Ethenone is highly poisonous; its toxicity is about eight times that of phosgene. Ethenone tends to spontaneously polymerize. Contact with hydrogen peroxide leads to an explosive reaction. It can form an explosive mixture with air. It is soluble in acetone, ethanol, ethyl ether, aromatic solvents and halocarbons.
Preparation
Ethenone was discovered at the same time by Hermann Staudinger and by Norman T. M. Wilsmore. Ethenone is produced on a large scale industrially for use in the production of acetic anhydride. It can be prepared by pyrolysis of acetone, and this was formerly the main industrial process. When passing acetone vapors through heated pipes or electrically heated metal wires at 500-600 °C in the presence of little carbon disulfide, acetone decomposes into methane and ethenone, with 95% yield. In industrial chemistry, ketone pyrolysis has largely been replaced by the dehydration of acetic acid.
Natural occurrence
Ethenone has been observed to occur in space, in comets or in gas as part of the interstellar medium.
Use
Ethenone is used to make acetic anhydride from acetic acid. Generally it is used for the acetylation of chemical compounds.
Ethenone reacts with methanal in the presence of catalysts such as Lewis acids to give β-propiolactone. The technically most significant use of ethenone is the synthesis of sorbic acid by reaction with 2-butenal in toluene at about 50 °C in the presence of zinc salts of long-chain carboxylic acids. This produces a polyester of 3-hydroxy-4-hexenoic acid, which is thermally or hydrolytically depolymerized to sorbic acid. Ethenone is very reactive, tending to react with nucleophiles to form an acetyl group. For example, it reacts with water to form acetic acid; with acetic acid to form acetic anhydride; with ammonia and amines to form ethanamides; and with dry hydrogen halides to form acetyl halides. The formation of acetic acid likely occurs by an initial formation of 1,1-dihydroxyethene, which then tautomerizes to give the final product. Ethenone will also react with itself via Woodward-Hoffmann rules| photocycloadditions to form cyclic dimers known as diketenes. For this reason, it should not be stored for long periods.
Hazards
Exposure to concentrated levels causes humans to experience irritation of body parts such as the eye, nose, throat and lungs. Extended toxicity testing on mice, rats, guinea pigs and rabbits showed that ten-minute exposures to concentrations of freshly generated ethenone as low as 0.2 mg/liter may produce a high percentage of deaths in small animals. These findings show ethenone is toxicologically identical to phosgene. The formation of ketene in the pyrolysis of vitamin E acetate, an additive of some e-liquid products, is one possible mechanism of the reported pulmonary damage caused by electronic cigarette use. Occupational exposure limits are set at 0.5 ppm over an eight-hour time-weighted average. An IDLH limit is set at 5 ppm, as this is the lowest concentration productive of a clinically relevant physiologic response in humans.
