Hydrogen bromide
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Names | |||
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Preferred IUPAC name
Hydrogen bromide[citation needed] | |||
Systematic IUPAC name
Bromane[1] | |||
Identifiers | |||
3D model (JSmol)
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3587158 | |||
ChEBI | |||
ChEMBL | |||
ChemSpider | |||
ECHA InfoCard | 100.030.090 | ||
EC Number |
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KEGG | |||
MeSH | Hydrobromic+Acid | ||
PubChem CID
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RTECS number |
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UN number | 1048 | ||
CompTox Dashboard (EPA)
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Properties | |||
BrH | |||
Molar mass | 80.912 g·mol−1 | ||
Appearance | Colorless gas | ||
Odor | Acrid | ||
Density | 3.307 g dm-3 | ||
Melting point | −87 °C; −125 °F; 186 K | ||
Boiling point | −66 °C; −87 °F; 207 K | ||
1.93 kg dm-3 (at 20 °C) | |||
Vapor pressure | 2.308 MPa (at 21 °C) | ||
Acidity (pKa) | ~–9 [2] | ||
Basicity (pKb) | ~23 | ||
Refractive index (nD)
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1.325 | ||
Structure | |||
Linear | |||
82 mD | |||
Thermochemistry | |||
Heat capacity (C)
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350.7 mJ K-1 g-1 | ||
Std molar
entropy (S⦵298) |
198.696-198.704 J K-1 mol-1[3] | ||
Std enthalpy of
formation (ΔfH⦵298) |
-36.45--36.13 kJ mol-1[3] | ||
Hazards | |||
GHS labelling: | |||
Danger | |||
H314, H335 | |||
P261, P280, P305+P351+P338, P310 | |||
NFPA 704 (fire diamond) | |||
Related compounds | |||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Hydrogen bromide is the diatomic molecule HBr. HBr is a gas at standard conditions. Hydrobromic acid forms upon dissolving HBr in water. Conversely, HBr can be liberated from hydrobromic acid solutions with the addition of a dehydration agent, but not by distillation. Hydrogen bromide and hydrobromic acid are, therefore, not the same, but they are related. Commonly, chemists refer to hydrobromic acid as "HBr", and this usage, while understood by most chemists, is imprecise and can be confusing to the non-specialist.
General description
At room temperature, HBr is a nonflammable gas with an acrid odor, fuming in moist air because of the formation of hydrobromic acid. HBr is very soluble in water, forming hydrobromic acid solution, which is saturated at 68.85% HBr by weight at room temperature. Aqueous solutions that are 47.38% HBr by weight form a constant-boiling mixture (reverse azeotrope) that boils at 126°C. Boiling less concentrated solutions releases H2O until the constant boiling mixture composition is reached.
Uses of HBr
There are many uses of HBr in chemical synthesis. For example, HBr is used for the production of alkyl bromides from alcohols:
- ROH + HBr → RBr + H2O
HBr adds to alkenes to give bromoalkanes, an important family of organobromine compounds:
- RCH=CH2 + HBr → RCH(Br)–CH3
HBr adds to alkynes to yield bromoalkenes. The stereochemistry of this type of addition is usually anti:
- RC≡CH + HBr → RC(Br)=CH2
HBr adds to the haloalkene to form a geminal dihaloalkane. (This type of addition follows Markovnikov's rule):
- RC(Br)=CH2 + HBr → RC(Br2)–CH3
Also, HBr is used to open epoxides and lactones and in the synthesis of bromoacetals. Additionally, HBr catalyzes many organic reactions.[4][5][6][7]
HBr has been proposed for use in a utility-scale flow-type battery.[8]
Industrial preparation
Hydrogen bromide (along with hydrobromic acid) is produced on a much smaller scale than the corresponding chlorides. In the primary industrial preparation, hydrogen and bromine are combined at temperatures between 200-400 °C. The reaction is typically catalyzed by platinum or asbestos.[5][9]
Laboratory synthesis
HBr can be synthesized by a variety of methods. It may be prepared in the laboratory by distillation of a solution of sodium or potassium bromide with phosphoric acid or diluted sulfuric acid[10]:
- 2 KBr + H2SO4 → K2SO4 + HBr
Concentrated sulfuric acid is ineffective because HBr formed will be oxidized to bromine gas:
- 2 HBr + H2SO4 → Br2 + SO2 + 2H2O
The acid may be prepared by several other methods, as well, including reaction of bromine either with phosphorus and water, or with sulfur and water[11]:
- 2 Br2 + S + 2 H2O → 4 HBr + SO2
Alternatively, it can be prepared by the bromination of tetraline (1,2,3,4-tetrahydronaphthalene):[12]
- C10H12 + 4 Br2 → C10H8Br4 + 4 HBr
Alternatively bromine can be reduced with phosphorous acid:[5]
- Br2 + H3PO3 + H2O → H3PO4 + 2 HBr
Anhydrous hydrogen bromide can also be produced on a small scale by thermolysis of triphenylphosphonium bromide in refluxing xylene.[4]
HBr prepared by the above methods can be contaminated with Br2, which can be removed by passing the gas through Cu turnings or through phenol.[9]
References
- ^ "Hydrobromic Acid - Compound Summary". PubChem Compound. USA: National Center for Biotechnology Information. 16 September 2004. Identification and Related Records. Retrieved 10 November 2011.
- ^ Perrin, D. D. Dissociation constants of inorganic acids and bases in aqueous solution. Butterworths, London, 1969.
- ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. ISBN 061894690X.
- ^ a b Hercouet, A.;LeCorre, M. (1988) Triphenylphosphonium bromide: A convenient and quantitative source of gaseous hydrogen bromide. Synthesis, 157-158.
- ^ a b c Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Butterworth-Heineman: Oxford, Great Britain; 1997; pp. 809-812.
- ^ Carlin, William W. U.S. patent 4,147,601, April 3, 1979
- ^ Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function; 4th Ed.; W. H. Freeman and Company: New York, NY; 2003.
- ^ http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/30535ag.pdf
- ^ a b Ruhoff, J. R.; Burnett, R. E.; Reid, E. E. "Hydrogen Bromide (Anhydrous)" Organic Syntheses, Vol. 15, p.35 (Coll. Vol. 2, p.338).
- ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
- ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
- ^ WebElements: Hydrogen Bromide