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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