Hypobromous acid

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Hypobromous acid
IUPAC name Hypobromous acid[note 1]
Other names Bromic(I) acid
Bromanol
Hydroxidobromine
Identifiers
InChI InChI=1/BrHO/c1-2/h2H
InChIKey CUILPNURFADTPE-UHFFFAOYAL
Standard InChI InChI=1S/BrHO/c1-2/h2H
Standard InChIKey CUILPNURFADTPE-UHFFFAOYSA-N
CAS number [13517-11-8]
ChemSpider 75379
Properties[2]
Chemical formula HOBr
Molar mass 96.911 g mol−1
Acidity (pKa) 8.80
Structure[3]
Molecular geometry bent: r(Br–O) = 182.80 pm, θ(Br–O–H) = 102.99°
Related compounds
Other hypohalous acids Hypofluorous acid
Hypochlorous acid
Hypoiodous acid
Other compounds Hydrobromic acid
Bromous acid
Bromic acid
Perbromic acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Hypobromous acid, HOBr, is an oxoacid of bromine. It is often considered to be only stable in aqueous solution,[4][5] but it can be prepared in sufficient quantities in the gas phase for spectroscopic studies.[3]

Preparation

Hypobromous acid can be prepared in solution by the reaction of bromine with a well-stirred suspension of mercury(II) oxide or with silver nitrate.[6]

2 Br2 + 2 HgO + H2O → 2 HOBr + HgO·HgBr2
Br2 + AgNO3 + H2O → HOBr + AgBr + HNO3

It is also formed by the disproportionation of bromine in water, although the equilibrium concentration is only 1.15 × 10−3 mol dm−3 in a saturated bromine solution (0.21 mol dm−3) at 25 °C.[5]

Br2 + H2O ⇌ HOBr + H+ + Br−

It can also be prepared by the reaction of water with dibromine monoxide, which is the formal anhydride of hypobromous acid.[2]

Br2O + H2O ⇌ 2 HOBr

Structure

Hypobromous acid has a bent C2v structure in the vapour phase, as expected from VSEPR theory, with r(Br–O) = 182.80 pm, θ(H–O–Br) = 102.99°.[3] Its rotational spectrum has been extensively studied.[7][8][9][10][11]

Reactivity

Hypobromous acid is unstable with respect to disproportionation, and should be prepared at around 0 °C.[5]

3 HOBr → 3 H+ + 2 Br− + BrO−3

The rate of disproportionation depends on the pH, being highest for pH 3–8.[12] The reaction is catalyzed by several species, including cobalt, nickel and copper ions[2] and phosphate and carbonate.[12]

Hypobromous acid will add to carbon–carbon double bonds to give bromohydrins,[13][14] and will also brominate activated aromatic rings such as toluene.[15]

The salts of hypobromous acid are called hypobromites. Because HOBr is only a very weak acid (pKa = 8.90), only salts of strong bases, such as the alkali and alkaline earth metal hydroxides, can be formed. Lithium, sodium and potassium hypobromites have well-defined solid hydrates.[16][17] Calcium hypobromite has been known as a bleaching agent since the early twentieth century, and probably has the formula CaBr(OBr).[2] Other hypobromites are likely to exist in solution.

Notes and references

Notes

  1. ↑ Hypobromous 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. ↑ 2.0 2.1 2.2 2.3 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.
  3. ↑ 3.0 3.1 3.2 Cohen, E. A.; Mcrae, G. A.; Tan, T. L.; Friedl, R. R.; Johns, J. W. C.; Noel, M. The ν1 Band of HOBr. J. Mol. Spectrosc. 1995, 173 (1), 55–61. DOI: 10.1006/jmsp.1995.1218.
  4. ↑ Greenwood, Norman N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; pp 999–1007. ISBN 0-08-022057-6.
  5. ↑ 5.0 5.1 5.2 Cotton, F. Albert; Wilkinson, Geoffrey Advanced Inorganic Chemistry, 5th ed.; Wiley-Interscience: New York, 1988; pp 563–67. ISBN 0-471-84997-9.
  6. ↑ 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, p 1400.
  7. ↑ Koga, Yoshinori; Takeo, Harutoshi; Kondo, Shigeo; Sugie, Masaaki; Matsumura, Chi; McRae, G. A.; Cohen, E. A. The rotational spectra, molecular structure, dipole moment, and hyperfine constants of HOBr and DOBr. J. Mol. Spectrosc. 1989, 138 (2), 467–81. DOI: 10.1016/0022-2852(89)90013-1.
  8. ↑ McRae, G. A.; Cohen, E. A. The ν2 band of HOBr. J. Mol. Spectrosc. 1990, 139 (2), 369–76. DOI: 10.1016/0022-2852(90)90074-Z.
  9. ↑ Orphal, J.; Kou, Q.; Kwabia Tchana, F.; Pirali, O.; Flaud, J.-M. The ν3 bands of HOBr around 16 μm measured by high-resolution Fourier-transform spectroscopy. J. Mol. Spectrosc. 2003, 221 (2), 239–43. DOI: 10.1016/S0022-2852(03)00222-4.
  10. ↑ Orphal, J.; Flaud, J.-M.; Kou, Q.; Kwabia Tchana, F.; Pirali, O. The far infrared rotational spectrum of HOBr: line positions and intensities. J. Mol. Struct. 2005, 742 (1–3), 153–59. DOI: 10.1016/j.molstruc.2005.01.006.
  11. ↑ Cohen, E. A.; Müller, H. S. P.; Tan, T. L.; McRae, G. A. High resolution spectroscopy of DOBr and molecular properties of hypobromous acid. J. Mol. Spectrosc. 2010, 262 (1), 30–36. DOI: 10.1016/j.jms.2010.04.009.
  12. ↑ 12.0 12.1 Beckwith, Richard C.; Margerum, Dale W. Kinetics of Hypobromous Acid Disproportionation. Inorg. Chem., 1997, 36 (17), 3754–60. DOI: 10.1021/ic970155g.
  13. ↑ Mokievsky Zh. Russ. Fiz.-Khim. O-va. 1898, 30, 900; Chem. Zentralbl. 1899, 1, 591.
  14. ↑ Ingold, Christopher Kelk; Smith, Henry Geoffrey CCCLXXXII.—The modes of addition to conjugated unsaturated systems. Part III. Further remarks and observations bearing on the mechanism of addition of halogens and hydrogen halides. J. Chem. Soc. 1931, 2752–65. DOI: 10.1039/JR9310002752.
  15. ↑ Stark, O. Über ein neues Bromierungsverfahren, Bromieren mit wäßriger, unterbromiger Säure. Ber. Dtsch. Chem. Ges. 1910, 43 (1), 670–74. DOI: 10.1002/cber.191004301113.
  16. ↑ Kelley, M. T.; LaValle, D. E.; Quincy, R. B. Inorganic Preparations. In Analytical Chemistry Division Annual Progress Report For Period Ending November 15, 1965; Raaen, Helen P., Ed.; Oak Ridge National Laboratory: Oak Ridge, Tenn., 1966; pp 71–73. ORNL-3889, <http://www.ornl.gov/info/reports/1966/3445605481993.pdf>.
  17. ↑ Scholder, R.; Krauss, K. Über Kristallisierte Alkalihypobromite. Z. Anorg. Allg. Chem. 1952, 268 (4–6), 279–90. DOI: 10.1002/zaac.19522680410.

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