Hypofluorous acid, chemical formula HOF, is the only known oxyacid of fluorine and the only known oxoacid in which the main atom gains electrons from oxygen to create a negative oxidation state. The oxidation state of the oxygen in this acid (and in the hypofluorite ion OF− and in its salts called hypofluorites) is 0, while its valence is 2. It is also the only hypohalous acid that can be isolated as a solid. HOF is an intermediate in the oxidation of water by fluorine, which produces hydrogen fluoride, oxygen difluoride, hydrogen peroxide, ozone and oxygen. HOF is explosive at room temperature, forming HF and O2:[1]
 Gas-phase structure
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| Names | |
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| IUPAC name
Hypofluorous acid
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Other names
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3D model (JSmol)
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PubChem CID
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CompTox Dashboard (EPA)
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| Properties | |
| HOF | |
| Molar mass | 36.0057 g/mol |
| Appearance | pale yellow liquid above −117 °C white solid below −117 °C |
| Melting point | −117 °C (−179 °F; 156 K) |
| Boiling point | decomposes at 0 °C (32 °F; 273 K)[citation needed] |
| Structure | |
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| Hazards | |
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Main hazards
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Explosive, strong oxidizer, corrosive |
| NFPA 704 (fire diamond) | |
| Related compounds | |
Other cations
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Lithium hypofluorite |
Related compounds
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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- 2 HOF → 2 HF + O2
This reaction is catalyzed by water.[2]
It was isolated in the pure form by passing F2 gas over ice at −40 °C, rapidly collecting the HOF gas away from the ice, and condensing it:[2]
- F2 + H2O → HOF + HF
The compound has been characterized in the solid phase by X-ray crystallography[1] as a bent molecule with an angle of 101°. The O–F and O–H bond lengths are 144.2 and 96.4 picometres, respectively. The solid framework consists of chains with O–H···O linkages. The structure has also been analyzed in the gas phase, a state in which the H–O–F bond angle is slightly narrower (97.2°).
Thiophene chemists commonly call a solution of hypofluorous acid in acetonitrile (generated in situ by passing gaseous fluorine through water in acetonitrile) Rozen's reagent.[3]
Difference from other hypohalous acids
editThe formal oxidation state of oxygen in hypofluorous acid and hypofluorite is 0; the same oxidation state found in molecular oxygen. In most oxygen compounds, including the other hypohalous acids, oxygen takes on a state of -2. The oxygen (0) atom is the root of hypofluorous acid's strength as an oxidizer, in contrast to the halogen (+1) atom in other hypohalic acids.
This alters the acid's chemistry. Where reduction of a general hypohalous acid reduces the halogen atom and yields the corresponding elemental halogen gas,
- 2 HOX + 2 H+ + 2 e− → 2 H2O + X2
reduction of hypofluorous acid instead reduces the oxygen atom and yields fluoride directly.
- HOF + H+ + 2 e− → H2O + F−
Unlike other hypohalous acids, HOF is a weaker oxidant than elemental fluorine.
Hypofluorites
editHypofluorites are formally derivatives of OF−, which is the conjugate base of hypofluorous acid. One example is trifluoromethyl hypofluorite (CF3OF), which is a trifluoromethyl ester of hypofluorous acid. The conjugate base is known in salts such as lithium hypofluorite.
See also
edit- Hypochlorous acid, a related compound that is more technologically important but has not been obtained in pure form.
References
edit- ^ a b W. Poll; G. Pawelke; D. Mootz; E. H. Appelman (1988). "The Crystal Structure of Hypofluorous Acid : Chain Formation by O-H · · · O Hydrogen Bonds". Angew. Chem. Int. Ed. Engl. 27 (3): 392–3. doi:10.1002/anie.198803921.
- ^ a b Appelman, Evan H. (1973-04-01). "Nonexistent compounds. Two case histories". Accounts of Chemical Research. 6 (4): 113–117. doi:10.1021/ar50064a001. ISSN 0001-4842.
- ^ For Rozen's original popularizations, see:
- Rozen, Shlomo (2001). "Hypofluorous Acid". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rh074. ISBN 0471936235.
- Rozen, Shlomo (2014). "HOF·CH3CN: Probably the Best Oxygen Transfer Agent Organic Chemistry Has To Offer". Acc. Chem. Res. 47 (8): 2378–2389. doi:10.1021/ar500107b. PMID 24871453.
- Singh, Raman; Kaur, Rajneesh; Gupta, Tarang; Kulbir, Kulbir; Singh, Kuldeep (2019). "Applications of Rozen's Reagent in Oxygen-Transfer and C-H Activation Reactions". Synthesis. 51 (2): 371–383. doi:10.1055/s-0037-1609638. S2CID 104572566.
- Dell, Emma J.; Campos, Luis M. (2012). "The preparation of thiophene-S,S-dioxides and their role in organic electronics". J. Mater. Chem. 22 (26): 12945–12952. doi:10.1039/C2JM31220D.