Hydrogen bromide
Hydrogen bromide is the diatomic molecule with the formula HBr. It is a colorless compound and a hydrogen halide. Hydrobromic acid is a solution of HBr in water. Both the anhydrous and aqueous solutions of HBr are common reagents in the preparation of bromide compounds.
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| Names | |||
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| Preferred IUPAC name
Hydrogen bromide | |||
| 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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| UNII | |||
| UN number | 1048 | ||
CompTox Dashboard (EPA) |
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| Properties | |||
| HBr | |||
| Molar mass | 80.91 g/mol | ||
| Appearance | Colorless gas | ||
| Odor | Acrid | ||
| Density | 3.6452 kg/m3 (0 °C, 1013 mbar)[2] | ||
| Melting point | −86.9 °C (−124.4 °F; 186.2 K) | ||
| Boiling point | −66.8 °C (−88.2 °F; 206.3 K) | ||
| 221 g/100 mL (0 °C) 204 g/100 mL (15 °C) 193 g/100 mL (20 °C) 130 g/100 mL (100 °C) | |||
| Solubility | Soluble in alcohol, organic solvents | ||
| Vapor pressure | 2.308 MPa (at 21 °C) | ||
| Acidity (pKa) | −8.8 (±0.8);[3] ~−9[4] | ||
| Basicity (pKb) | ~23 | ||
| Conjugate acid | Bromonium | ||
| Conjugate base | Bromide | ||
Refractive index (nD) |
1.325 | ||
| Structure | |||
| Linear | |||
| 820 mD | |||
| Thermochemistry | |||
Heat capacity (C) |
350.7 mJ/(K·g) | ||
Std molar entropy (S |
198.696–198.704 J/(K·mol)[5] | ||
Std enthalpy of formation (ΔfH⦵298) |
−36.45...−36.13 kJ/mol[5] | ||
| Hazards | |||
| Safety data sheet | hazard.com | ||
| GHS pictograms | |||
| GHS Signal word | Danger | ||
| H314, H335 | |||
| P261, P280, P305+351+338, P310 | |||
| NFPA 704 (fire diamond) | |||
| Lethal dose or concentration (LD, LC): | |||
LC50 (median concentration) |
2858 ppm (rat, 1 h) 814 ppm (mouse, 1 h)[6] | ||
| NIOSH (US health exposure limits): | |||
PEL (Permissible) |
TWA 3 ppm (10 mg/m3)[7] | ||
REL (Recommended) |
TWA 3 ppm (10 mg/m3)[7] | ||
IDLH (Immediate danger) |
30 ppm[7] | ||
| Related compounds | |||
Related compounds |
Hydrogen fluoride Hydrogen chloride Hydrogen iodide Hydrogen astatide | ||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |||
| Infobox references | |||
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 47.6% HBr by mass form a constant-boiling azeotrope mixture that boils at 124.3 °C. Boiling less concentrated solutions releases H2O until the constant-boiling mixture composition is reached.
Uses of HBr
Hydrogen bromide and hydrobromic acid are important reagents in the production of inorganic and organic bromine compounds.[8] The free-radical addition of HBr to alkenes gives alkyl bromides:
- RCH=CH2 + HBr → R−CHBr−CH3
These alkylating agents are precursors to fatty amine derivatives. Similar free radical addition to allyl chloride and styrene gives 1-bromo-3-chloropropane and phenylethylbromide, respectively.
Hydrogen bromide reacts with dichloromethane to give bromochloromethane and dibromomethane, sequentially:
- HBr + CH2Cl2 → HCl + CH2BrCl
- HBr + CH2BrCl → HCl + CH2Br2
Allyl bromide is prepared by treating allyl alcohol with HBr:
- CH2=CHCH2OH + HBr → CH2=CHCH2Br + H2O
Other reactions
Although not widely used industrially, HBr adds to alkenes to give bromoalkanes, an important family of organobromine compounds. Similarly, HBr adds to haloalkene to form a geminal dihaloalkane. (This type of addition follows Markovnikov's rule):
- RC(Br)=CH2 + HBr → RC(Br2)−CH3
HBr also adds to alkynes to yield bromoalkenes. The stereochemistry of this type of addition is usually anti:
- RC≡CH + HBr → RC(Br)=CH2
Also, HBr is used to open epoxides and lactones and in the synthesis of bromoacetals. Additionally, HBr catalyzes many organic reactions.[9][10][11][12]
Potential applications
HBr has been proposed for use in a utility-scale flow-type battery.[13]
Industrial preparation
Hydrogen bromide (along with hydrobromic acid) is produced by combining hydrogen and bromine at temperatures between 200 and 400 °C. The reaction is typically catalyzed by platinum or asbestos.[10][14]
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 bromide or potassium bromide with phosphoric acid or sulfuric acid:[15]
- KBr + H2SO4 → KHSO4 + HBr
Concentrated sulfuric acid is less effective because it oxidizes HBr to bromine:
- 2 HBr + H2SO4 → Br2 + SO2 + 2 H2O
The acid may be prepared by:
- reaction of bromine with water and sulfur:[15]
- 2 Br2 + S + 2 H2O → 4 HBr + SO2
- bromination of tetralin:[15]
- C10H12 + 4 Br2 → C10H8Br4 + 4 HBr
- reduction of bromine with phosphorous acid:[10]
- 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.[9]
Hydrogen bromide prepared by the above methods can be contaminated with Br2, which can be removed by passing the gas through a solution of phenol at room temperature in tetrachloromethane or other suitable solvent (producing 2,4,6-tribromophenol and generating more HBr in the process) or through copper turnings or copper gauze at high temperature.[14]
Safety
HBr is highly corrosive and irritating to inhalation.
References
- "Hydrobromic Acid - Compound Summary". PubChem Compound. USA: National Center for Biotechnology Information. 16 September 2004. Identification and Related Records. Retrieved 10 November 2011.
- Record in the GESTIS Substance Database of the Institute for Occupational Safety and Health
- Trummal, Aleksander; Lipping, Lauri; Kaljurand, Ivari; Koppel, Ilmar A; Leito, Ivo (2016). "Acidity of Strong Acids in Water and Dimethyl Sulfoxide". The Journal of Physical Chemistry A. 120 (20): 3663. doi:10.1021/acs.jpca.6b02253. PMID 27115918.
- Perrin, D. D. Dissociation constants of inorganic acids and bases in aqueous solution. Butterworths, London, 1969.
- Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. ISBN 0-618-94690-X.
- "Hydrogen bromide". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
- NIOSH Pocket Guide to Chemical Hazards. "#0331". National Institute for Occupational Safety and Health (NIOSH).
- Dagani, M. J.; Barda, H. J.; Benya, T. J.; Sanders, D. C. "Bromine Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a04_405.CS1 maint: multiple names: authors list (link)
- Hercouet, A.; LeCorre, M. (1988) Triphenylphosphonium bromide: A convenient and quantitative source of gaseous hydrogen bromide. Synthesis, 157–158.
- 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
- Ruhoff, J. R.; Burnett, R. E.; Reid, E. E. "Hydrogen Bromide (Anhydrous)" Organic Syntheses, Vol. 15, p. 35 (Coll. Vol. 2, p. 338).
- M. Schmeisser "Chlorine, Bromine, Iodine" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 282.