Ketene

Ketene (systematic name ethenone) is a colorless, toxic gas with a “penetrating” odor, according to the Merck Index. It is soluble in essentially all organic solvents, but it decomposes in water to form acetic acid. Its tautomer, ethynol, is known to exist only by photoisomerizing ketene in an argon matrix. Nobel Prize–winning German chemist Hermann Staudinger discovered the ketene family of organic compounds early in the 20th century.

Ketene is prepared by pyrolyzing acetone, acetic acid, or acetic anhydride or by treating acetyl chloride with a nonprotic nucleophile. It is useful for acetylating nucleophiles to make esters, amides, and other compounds that cannot easily be made with other reagents. Ketenes were used in the synthesis of the early antibiotics penicillin and amoxicillin.

The anions cyanate (NCO^–) and 2-phosphaethynolate (PCO^–) are isoelectronic with ketene. Now Alexander Hinz and Jose M. Goicoechea at Oxford University (UK) report the preparation of the arsenic analogue, 2-arsaethynolate (AsCO^–) via a three step sequence that starts from elemental arsenic. AsCO^– readily reacts with ketenes and carbodiimides to form 4-membered arsenic heterocycles.