Hypohalous acid

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.

Stability

Hypohalous acids tend to be unstable. Only hypofluorous acid has been isolated as a solid, and even it is explosive at room temperature.[1] Hypochlorous acid cannot be prepared in anhydrous form.[2] Hypobromous acid, hypoiodous acid, and their conjugate bases (hypobromite and hypoiodite) are also unstable, undergoing disproportionation reactions like

3 BrO−(aq) → 2 Br−(aq) + BrO−3(aq)

and

3 HIO → 2 HI + HIO3

that result in the corresponding hydrogen halides/halide ions and halic acids/halates.[3]

Uses

Hypochlorous acid and hypobromous acid are each dissolved in water in order to sanitize it, hypochlorous acid in swimming pools and hypobromous acid in hot tubs and spas.[4]

Acidity

Hypohalous acids tend to be weak acids, and they tend to get weaker as the halogen progresses farther down the periodic table. Hypochlorous acid has a pKa of 7.53.[5] The pKa values of hypobromous acid is higher (meaning that it is an even weaker acid),[6] at 8.65. The pKa of hypoiodous acid is even higher, at 10.6.[7]

References
  1. 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.
  2. Inorganic chemistry, Egon Wiberg, Nils Wiberg, Arnold Frederick Holleman, "Hypochlorous acid" p.442, section 4.3.1
  3. Holleman, A. F.; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Academic Press. p. 451. ISBN 9780123526519. Retrieved 24 September 2018.
  4. 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.
  5. Harris, Daniel C. (2009). Exploring Chemical Analysis (Fourth ed.). p. 538.
  6. Holleman, A. F.; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Academic Press. p. 449. ISBN 9780123526519. Retrieved 7 October 2018.
  7. "Oxoacids | Introduction to Chemistry". courses.lumenlearning.com. Retrieved 26 February 2019. HOCl pKa = 7.5 < HOBr pKa = 8.6 < HOI pKa = 10.6
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