A hypohalous acid is an oxyacid consisting of a hydroxyl group single-bonded to any halogen. Examples include hypofluorous acid, hypochlorous acid, hypobromous acid, and hypoiodous acid. The conjugate base is a hypohalite. They can be formed by reacting the corresponding diatomic halogen molecule (F2, Cl2, Br2, I2) with water in the reaction:

X2 + H2O <→ HXO + HX

This also results in the corresponding hydrogen halide, which is also acidic. A pseudohalogen would instead generate a hypopseudohalous acid.

It is uncertain that whether hypoastatous acid (AtOH), the hypohalous acid for astatine, is more similar to other hypohalous acids or to metal hydroxides.[1]

Stability

Hypohalous acids tend to be unstable. Only hypofluorous acid has been isolated as a solid, and even it is explosive at room temperature.[2] Hypochlorous acid cannot be prepared in anhydrous form.[3] Hypobromous acid, hypoiodous acid, and their conjugate bases (hypobromite and hypoiodite) are also unstable, undergoing disproportionation reactions like

3 BrO-(aq) → 2 Br-(aq) + BrO3−(aq)

and

3 HIO → 2 HI + HIO3

that result in the corresponding hydrogen halides/halide ions and halic acids/halates.[4]

Uses

Hypochlorous acid and hypobromous acid are each dissolved in water to sanitize it, hypochlorous acid in swimming pools and hypobromous acid in hot tubs and spas.[5]

Acidity

Hypohalous acids tend to be weak acids, and they typically get weaker as the halogen progresses further down the periodic table. Hypochlorous acid has a pKa of 7.53.[6] The pKa values of hypobromous acid is higher (meaning that it is an even weaker acid),[7] at 8.65. The pKa of hypoiodous acid is even higher, at 10.6.

References

  1. ^ Chiera, Nadine Mariel et al. (2025-03-19). "Observation of a volatile astatine hydroxide species in online gas-adsorption thermochromatography experiments". Molecular Physics. 123 (5-6). doi:10.1080/00268976.2023.2272685. hdl:20.500.11850/643703. ISSN 0026-8976
  2. ^ 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
  3. ^ Inorganic chemistry, Egon Wiberg, Nils Wiberg, Arnold Frederick Holleman, "Hypochlorous acid" p.442, section 4.3.1
  4. ^ Holleman, A. F.; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Academic Press. p. 451. ISBN 9780123526519. Retrieved 24 September 2018.
  5. ^ Gonick, Larry & Criddle, Craig (2005-05-03). "Chapter 9 Acid Basics". The cartoon guide to chemistry. 1st ed. HarperResource. p. 189. ISBN 9780060936778. Similarly, we add HOCl to swimming pools to kill bacteria.
  6. ^ Harris, Daniel C. (2009). Exploring Chemical Analysis. Fourth ed. p. 538
  7. ^ Holleman, A. F.; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Academic Press. p. 449. ISBN 9780123526519. Retrieved 7 October 2018.