Fulminic acid
IUPAC name
3D model (JSmol)
  • InChI=1S/CHNO/c1-2-3/h1H checkY
  • InChI=1/CHNO/c1-2-3/h1H
  • [O-][N+]#C
Molar mass 43.02 g mol−1
Conjugate base Fulminate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Fulminic acid is an acid with the formula HCNO, more specifically H−C≡N+−O. It is an isomer of isocyanic acid (H−N=C=O) and of its elusive tautomer, cyanic acid (H−O−C≡N), and also of isofulminic acid (H−O−N+≡C).[1]

Fulminate is the anion [C≡N+−O] or any of its salts. For historical reasons, the fulminate functional group is understood to be −O−N+≡C as in isofulminic acid;[2] whereas the group −C≡N+O is called nitrile oxide.


This chemical was known since the early 1800s through its salts and via the products of reactions in which it was proposed to exist,[3] but the acid itself was not detected until 1966.[1]


Fulminic acid was long believed to have a structure of H–O–N+≡C. It wasn't until the 1966 isolation and analysis of a pure sample of fulminic acid that this structural idea was conclusively disproven.[3] The chemical that actually has that structure, isofulminic acid (a tautomer of the actual fulminic acid structure) was eventually detected in 1988.[3]

The structure of the molecule has been determined by microwave spectroscopy with the following bond-lengths - C-H: 1.027(1)Å, C-N: 1.161(15)Å, N-O: 1.207(15)Å.[4]


A convenient synthesis involves flash pyrolysis of certain oximes. In contrast to earlier syntheses, this method avoids the use of highly explosive metal fulminates.[5]


  1. ^ a b Beck, W.; Feldl, K. (1966). "The Structure of Fulminic Acid, HCNO". Angew. Chem. Int. Ed. Engl. 5 (8): 722–723. doi:10.1002/anie.196607221.
  2. ^ Wentrup, Curt; Gerecht, Bernd; Laqua, Dieter; Briehl, Horst; Winter, Hans Wilhelm; Reisenauer, Hans Peter; Winnewisser, Manfred (1981). "Organic fulminates, R-O-NC". Journal of Organic Chemistry. 46 (5): 1046–1048. doi:10.1021/jo00318a050.
  3. ^ a b c Kurzer, Frederick (2000). "Fulminic Acid in the History of Organic Chemistry". Journal of Chemical Education. 77 (7): 851–857. Bibcode:2000JChEd..77..851K. doi:10.1021/ed077p851.
  4. ^ Winnewisser, Manfred; Bodenseh, Hans Karl (1967). "Mikrowellenspektrum, Struktur und /-Typ-Dublett-Aufspaltung der HCNO (Knallsäure)". Z. Naturforsch. 22 a (11): 1724–1737. Bibcode:1967ZNatA..22.1724W. doi:10.1515/zna-1967-1109. S2CID 96725880.
  5. ^ Wentrup, Curt; Gerecht, Bernd; Horst, Briehl (1979). "A New Synthesis of Fulminic Acid". Angew. Chem. 18 (6): 467–468. doi:10.1002/anie.197904671.