Bromous acid

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Bromous acid
IUPAC name Bromous acid[note 1]
Other names Bromic(III) acid
Hydroxy-λ3-bromanone
Hydroxidooxidobromine
Identifiers
InChI InChI=1/BrHO2/c2-1-3/h(H,2,3)
InChIKey DKSMCEUSSQTGBK-UHFFFAOYAC
Standard InChI InChI=1S/BrHO2/c2-1-3/h(H,2,3)
Standard InChIKey DKSMCEUSSQTGBK-UHFFFAOYSA-N
CAS number [37691-27-3]
ChemSpider 145144
Properties[2][3]
Chemical formula HBrO2
Molar mass 112.91 g mol−1
Acidity (pKa) 3.43(11)
Related compounds
Other halous acids Chlorous acid
Iodous acid
Other compounds Hydrobromic acid
Hypobromous acid
Bromic acid
Perbromic acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Bromous acid, HBrO2, is an oxoacid of bromine. Its existence as a molecular species has been considered doubtful[4][5] but, although it is unstable with respect to disproportionation, its lifetime in aqueous solution is sufficient for spectroscopic study.[3][6] It is an intermediate in the Belousov–Zhabotinskii reaction.[6][7][8][9][10] Some salts (bromites) are known.[4]

Disproportionation

Bromous acid is unstable with respect to disproportionation: at acid pH, the thermodynamic products are bromine and bromate.

5 HBrO2 → Br2 + 3 BrO−3 + 3 H+ + H2O

The initial mechanism is simpler than that for chlorous acid, as the production of bromine dioxide and the oxidation of water to oxygen are negligible. Hence, there are just two significant initial steps:[3]

HBrO2 + HBrO2 → HBrO + BrO−3 + H+       k = 800(100) dm3 mol−1 s−1
HBrO2 + BrO−2 → HBrO + BrO−3       k = 39.1(26) dm3 mol−1 s−1

The hypobromous acid then disproportionates further to give the final products. There is no evidence of any catalysis of the disporportionation of HBrO2 by bromide ions (unlike the case of chlorous acid and chloride ions).[3][11]

Acidity

pKa
HClO
7.47 		HBrO
8.80 		HIO
10.70
HClO2
1.96 		HBrO2
3.43 		HIO2
—
HClO3
< 0 		HBrO3
< 0 		HIO3
0.80

The acid dissociation constant of bromous acid can be calculated from the observed kinetics of its disproportionation:[3] Ka = 3.7(9) × 10−4 (pKa = 3.43). This is consistent with that which would be expected from periodic trends.

Hence, bromous acid would be expected to form salts with a wide range of metal ions. Nevertheless, only the bromites of the alkali and alkaline earth metals are well characterized.[4] They are not prepared by the neutralization of bromous acid, but by the controlled disproportionation of the corresponding hypobromite at pH 11–12 and 0 °C[12][13][14] or by heating the corresponding bromate in vacuo.[15] By comparison with the corresponding chlorites, the bromites of less electropositive cations might be expected to be explosively unstable.[16]

Bromous acid itself may be prepared by acidifying a solution of a bromite.[3] It is also thought to be produced in small amounts by the photolysis of bromates, particularly in the Belousov–Zhabotinskii reaction.

Notes and references

Notes

  1. ↑ Bromous acid is a retained name in IUPAC nomenclature.[1]

References

  1. ↑ Nomenclature of Inorganic Chemistry; IUPAC Recommendations 2005; Royal Society of Chemistry: Cambridge, 2005; p 287. ISBN 0-85404-438-8, <http://www.iupac.org/publications/books/rbook/Red_Book_2005.pdf>.
  2. ↑ Faria, Roberto de Barros; Epstein, Irving R.; Kustin, Kenneth Systematic design of chemical oscillators. Part 84. Determination of the pKa of bromous acid. J. Phys. Chem. 1992, 96 (17), 6861–63. DOI: 10.1021/j100196a003.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Faria, R. B.; Epstein, Irving R.; Kustin, Kenneth Kinetics of Disproportionation and pKa of Bromous Acid. J. Phys. Chem. 1994, 98 (4), 1363–67. DOI: 10.1021/j100055a051.
  4. ↑ 4.0 4.1 4.2 Ukeles, S. D.; Freiberg, M. Bromine, Inorganic Compounds. In Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley: New York, 2002. DOI: 10.1002/0471238961.021815131001031.
  5. ↑ Cotton, F. Albert; Wilkinson, Geoffrey Advanced Inorganic Chemistry, 5th ed.; Wiley-Interscience: New York, 1988; pp 566–67. ISBN 0-471-84997-9.
  6. ↑ 6.0 6.1 Ariese, Freek; Ungvarai-Nagy, Zsuzsanna The disproportionation of bromous acid (HBrO2), key species of the Belousov-Zhabotinskii oscillating reaction. J. Phys. Chem. 1986, 90 (1), 1–4. DOI: 10.1021/j100273a001.
  7. ↑ Sullivan, James C.; Thompson, Richard C. Kinetic study of the cerium(IV)-bromous acid reaction in acid sulfate solution. Implications for the Belousov-Zhabotinskii oscillating reaction. Inorg. Chem. 1979, 18 (9), 2375–79. DOI: 10.1021/ic50199a009.
  8. ↑ Hynne, F.; Graae Soerensen, P.; Neergaard, H. Oscillations of bromous acid, hypobromous acid, bromide, and cerium(4+) concentrations in the Belousov-Zhabotinskii reaction reconstructed from quenching experiments. J. Phys. Chem. 1991, 95 (3), 1315–18. DOI: 10.1021/j100156a052.
  9. ↑ Ruoff, Peter; Foersterling, Horst Dieter; Gyorgyi, Laszlo; Noyes, Richard M. Bromous acid perturbations in the Belousov-Zhabotinskii reaction: experiments and model calculations of phase response curves. J. Phys. Chem. 1991, 95 (23), 9314–20. DOI: 10.1021/j100176a052. Foersterling, Horst Dieter; Varga, Margit Bromous acid/cerium(4+): reaction and HBrO2 disproportionation measured in sulfuric acid solution at different acidities. J. Phys. Chem. 1993, 97 (30), 7932–38. DOI: 10.1021/j100132a022.
  10. ↑ Pelle, Krisztina; Wittmann, Maria; Lovrics, Klára; Noszticzius, Zoltán; Turco Liveri, Maria L.; Lombardo, Renato Mechanistic Investigations of the BZ Reaction with Oxalic Acid Substrate. I. The Oscillatory Parameter Region and Rate Constants Measured for the Reactions of HOBr, HBrO2, and Acidic BrO3− with Oxalic Acid. J. Phys. Chem. A 2004, 108 (25), 5377–85. DOI: 10.1021/jp048817s. Onel, Lavinia; Bourceanu, Gelu; Bitter, István; Wittmann, Mária; Noszticzius, Zoltán Uncatalyzed Reactions in the Classical Belousov−Zhabotinsky System. 2. The Malonic Acid−Bromate Reaction in Acidic Media. J. Phys. Chem. A 2006, 110 (3), 990–96. DOI: 10.1021/jp055259o.
  11. ↑ Ariese, Freek; Nagy, Zsuzsanna The bromide/bromous acid reaction. Switch-controlling step of the Belousov-Zhabotinskii oscillating system. J. Phys. Chem. 1986, 90 (8), 1496–98. DOI: 10.1021/j100399a005.
  12. ↑ Kircher, R.; Periat, R. Preparation du bromite de baryum cristallisé. Ind. Chim. Belge, Suppl. 1 1959, 877.
  13. ↑ Kircher, René; Periat, Robert (Société d'Etudes Chimiques pour l'Industrie et l'Agriculture) Verfahren zur Herstellung von Bariumbromit durch Disproportionierung von Bariumhypobromit. DE Patent 1076096, published 25 February 1960.
  14. ↑ Massagli, A.; Indelli, A.; Pergola, F. Kinetic investigation of the decomposition of bromite. Inorg. Chim. Acta 1970, 4 (4), 593–96. DOI: 10.1016/S0020-1693(00)93357-7.
  15. ↑ Downs, A. J.; Adams, C. J. In Comprehensive Inorganic Chemistry; Bailar, J. C., Jr.; Emeleus, H. J.; Nyholm, R., et al., Eds.; Pergamon: Oxford, 1973; Vol. 2, pp 1419–20.
  16. ↑ Greenwood, Norman N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; pp 1007–9. ISBN 0-08-022057-6.

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