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 pK_a of 7.53.^[5] The pK_a values of hypobromous acid is higher (meaning that it is an even weaker acid),^[6] at 8.65. The pK_a of hypoiodous acid is even higher, at 10.6.^[7]

References

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

[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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