Acetylenedicarboxylic acid
Acetylenedicarboxylic acid or butynedioic acid is an organic compound (a dicarboxylic acid) with the formula C4H2O4 or HO2C-C≡C-CO2H. It is a crystalline solid that is soluble in diethyl ether.
The removal of two protons yields the acetylenedicarboxylate dianion C4O4, which consists only of carbon and oxygen, making it an oxocarbon anion. Partial ionization yields the monovalent hydrogenacetylenedicarboxylate anion HC4O4.