Carbon Suboxide, C₃O₂

Carbon Suboxide, C₃O₂, is among the products of the distillation of ethyl malonate at 300° C. and a pressure of 12 mm. over a large excess of phosphoric oxide. Other malonic esters and the esters of related acids, as well as malonic acid itself, can similarly be made to yield the suboxide by a reaction which is essentially

CH₂(COOH)₂ = C₃O₂ + 2H₂O.

The suboxide is freed from ethylene and carbon dioxide, formed at the same time, by allowing the distillate, condensed by liquid air, to boil at atmospheric temperature; the residue is then vaporised, and collected at -60° C. Carbon suboxide is also formed by the interaction of malonyl chloride with silver, lead, or zinc oxide, thus:

CH₂(COCl)₂ + Ag₂O = C₃O₂ + 2AgCl + H₂O.

Silver oxalate or malonate may be employed in place of the oxide. Carbon suboxide is a colourless, mobile, refractive liquid having an odour which resembles that of acraldehyde and mustard oil, and is poisonous. It boils at 7° C. under 761 mm. pressure, and its vapour density corresponds to the formula C₃O₂. It burns with a bright blue smoky flame, producing carbon dioxide according to the reaction: C₃O₂ + 2O₂ = 3CO₂. The liquid oxide slowly changes at atmospheric temperature into a dark red solid of approximately the same composition, whose aqueous solution is eosin-red. This change, which is partly one of decomposition, is rapid at 37° C. and instantaneous at 100° C. The oxide combines with water to form malonic acid, and with hydrogen chloride, ammonia, and aniline to form malonyl chloride, malonamide, and malonanilide respectively. Its discoverers therefore regarded it as the second anhydride of malonic acid, thus:

→ →

Michael considers the oxide to be the lactone of β-hydroxy-propriolic acid, and thus to have the constitution ; but this view is not acceptable to Diels and Meyerheim, who adhere to the "dioxoallene" formula, O:C:C:C:O; and Diels and Lalin have shown that the suboxide behaves as a ketene, giving with formic acid the additive compound: .

Berthelot claimed that the term "carbon suboxide" is properly applied to the compounds discovered previously by Brodie and himself, and therefore cannot legitimately be used for the compound C₃O₂.