 In the presence of certain acids, including sulfuric and hydrofluoric acids, however, a variant of the Koch reaction occurs instead, and formic acid adds to the alkene to produce a larger carboxylic acid.An unstable formic anhydride, H(C=O)−O−(C=O)H, can be obtained by dehydration of formic acid with "N","N′"-dicyclohexylcarbodiimide in ether at low temperature.Some alchemists and naturalists were aware that ant hills give off an acidic vapor as early as the 15th century.