Dioxane tetraketone

Dioxane tetraketone (C₄O₆) is a chemical curiosity. Also known as dimeric oxalic anhydride, it’s an oxide of carbon; but unlike carbon monoxide and carbon dioxide, it’s difficult to make and impossible to isolate.

Paolo Strazzolini and colleagues at the University of Udine (Italy) and the University of Zagreb (Croatia) synthesized C₄O₆ in 1998. They combined oxalyl chloride (or bromide) with a suspension of silver oxalate in diethyl ether at –15 to –10 ºC for 1 h, then, also at low temperature, filtered the unreacted silver salt and evaporated the unreacted dihalide and solvent to leave a white solid.

The authors redissolved the product in CDCl₃–Et₂O and obtained a ¹³C NMR spectrum that revealed a single peak (other than the ether peaks) at 144.94 ppm. They concluded that the only plausible interpretation of the spectrum was C₄O₆, which has a calculated peak of 154 ppm.

C₄O₆ is stable in ether solution at –30 ºC. When the solution is warmed to 0 ºC, the compound decomposes to CO and CO₂. Treating C₄O₆ with methanol gives the mono- and dimethyl esters of oxalic acid. With diazomethane, the products are dimethyl oxalate and its mono- and bis(diazo) derivatives.

More than two dozen other carbon oxides, ranging from CO₃ to C₁₂O₁₂, and with varying degrees of stability, have been reported.