Hypofluorous acid, chemical formula HOF, is the only known oxoacid of fluorine and the only known oxoacid which the main atom gains electrons from oxygen to create a negative oxidation state. The oxidation state of the oxygen in hypofluorites is 0. 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:
3D model (JSmol)
|Molar mass||36.0057 g mol−1|
|Appearance||pale yellow liquid above −117 °C|
white solid below −117 °C
|Melting point||−117 °C (−179 °F; 156 K)|
decomposes at 0 °C 
|Main hazards||strong oxidizer, corrosive|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
- 2 HOF → 2 HF + O2
It was isolated in the pure form by passing F2 gas over ice at −40 °C, collecting the HOF gas, and condensing it:
- F2 + H2O → HOF + HF
The compound has been characterized in the solid phase by X-ray crystallography 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°).
- Hypochlorous acid, a related compound that is more technologically important but has not been obtained in pure form.
- 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.
- Rozen, Shlomo (2001). "Hypofluorous Acid". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rh074.
- Rozen, Shlomo (2014). "HOF·CH3CN: Probably the Best Oxygen Transfer Agent Organic Chemistry Has To Offer". Acc. Chem. Res. 47: 2378–2389. doi:10.1021/ar500107b.
- 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: 371–383. doi:10.1055/s-0037-1609638.