Formic anhydride, also called methanoic anhydride, is an organic compound with the chemical formula and a structural formula of (H(C=O)−)2O. It can be viewed as the anhydride of formic acid (HCOOH).
Formic anhydride can be obtained by reaction of formyl fluoride with excess sodium formate and a catalytic amount of formic acid in ether at −78 °C.[1] It can also be produced by reacting formic acid with N,-dicyclohexylcarbodiimide ((−N=)2C) in ether at −10 °C.[2] It can also be obtained by disproportionation of acetic formic anhydride.[3]
Formic anhydride is a liquid with boiling point 24 °C at 20 mmHg.[2] It is stable in diethyl ether solution. It can be isolated by low-temperature, low-pressure distillation, but decomposes on heating above room temperature.[2] At room temperature and higher, it decomposes through a decarbonylation reaction into formic acid and carbon monoxide.[4] Due to its instability, formic anhydride is not commercially available and must be prepared fresh and used promptly.
The decomposition of formic anhydride may be catalyzed by formic acid.[2]
Formic anhydride can be detected in the gas-phase reaction of ozone with ethylene.[5] The molecule is planar in the gas phase.[2]