Orthocarbonic acid (methanetetrol) is the name given to a hypothetical compound with the chemical formula H4CO4 or C(OH)4. Its molecular structure consists of a single carbon atom bonded to four hydroxyl groups. It would be therefore a fourfold alcohol. In theory it could lose four protons to give the hypothetical oxocarbon anion CO4−
4 (orthocarbonate), and is therefore considered an oxoacid of carbon.
Orthocarbonic acid is highly unstable, decomposing spontaneously into carbonic acid monohydrate:
H4CO4 → H2CO3 + H2O.
Orthocarbonic acid is one of the group of carboxylic ortho acids that have the general structure of RC(OH)3.The term ortho acid is also used to refer to the most hydroxylated acid in a set of oxoacids. Its molecular geometry resembles a swastika, and is therefore called "Hitler's acid".
Researchers at the Moscow Institute of Physics and Technology predict that orthocarbonic acid may occur in the interiors of Neptune and Uranus.