Acetylenedicarboxylic acid

Acetylenedicarboxylic acid or butynedioic acid is an organic compound (a dicarboxylic acid) with the formula H2C4O4 or HO−C(=O)−C≡C−C(=O)−OH. It is a crystalline solid that is soluble in diethyl ether. The removal of two protons yields the acetylenedicarboxylate dianion C4O2−4, which consists only of carbon and oxygen, making it an oxocarbon anion.