Hydrogen bromide
1114337
225369651
2008-07-13T09:31:12Z
Tamatisk
1093759
Reverted edits by [[Special:Contributions/60.50.205.211|60.50.205.211]] ([[User talk:60.50.205.211|talk]]) using [[User:MichaelBillington/MWT|MWT]]
{{Chembox new
| Name = Hydrogen bromide
| ImageFile = Hydrogen-bromide-2D-dimensions.png
<!-- | ImageSize = 120px -->
| ImageName = Hydrogen bromide
| ImageFile1 = Hydrogen-bromide-3D-vdW.png
<!-- | ImageSize1 = 120px -->
| ImageName1 = Hydrogen bromide
| PubChem = 260
| IUPACName = Hydrogen Bromide
| OtherNames =
| Section1 = {{Chembox Identifiers
| CASNo = 10035-10-6
}}
| Section2 = {{Chembox Properties
| Formula = HBr
| MolarMass = 80.912 g/mol
| Appearance = Colorless gas.
| Density = 3.307 g/L, gas.
| Solubility = 193 g/100 ml (20 °C)
| MeltingPt = –86.80°C (186.35 K)
| BoilingPt = –66.38°C (206.77 K)
| pKa = ≈ –9
}}
| Section3 = {{Chembox Structure
| MolShape = Linear.
| Dipole = 0.82 [[Debye|D]]
}}
| Section7 = {{Chembox Hazards
| ExternalMSDS = [http://www.jtbaker.com/msds/englishhtml/H3768.htm hydrobromic acid]<br /> [http://physchem.ox.ac.uk/MSDS/HY/hydrogen_bromide.html hydrogen bromide]
| MainHazards = Toxic, corrosive.
| {{NFPA 704}}
| NFPA-H = 4
| NFPA-F = 0
| NFPA-R = 2
| NFPA-O = COR
}}
}}
'''Hydrogen bromide''' is the diatomic molecule [[Hydrogen|H]][[Bromine|Br]]. Under standard conditions, HBr is a gas, but it can be liquified. The [[aqueous]] solution [[hydrobromic acid]] forms upon dissolving HBr in water. Conversely, HBr can be liberated from hydrobromic acid solutions upon the addition of a dehydration agents. 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. Hydrobromic acid is almost completely dissociated into H<sup>+</sup> and Br<sup>–</sup> in aqueous solution. Aqueous solutions that are 47.38% HBr by weight form a constant-boiling mixture (reverse [[azeotrope]]) that boils at 126°C. Boiling a solutions less concentrated causes H<sub>2</sub>O to boil in excess until the constant boiling mixture 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 → R<sup>+</sup>OH<sub>2</sub> + Br<sup>–</sup> → RBr + H<sub>2</sub>O
HBr adds to [[alkenes]] to give bromoalkanes:
::RCH=CH<sub>2</sub> + HBr → RCH(Br)–CH<sub>3</sub>
HBr adds to [[alkynes]] to yield haloalkenes. The [[stereochemistry]] of this type of addition is usually ''anti''):
:RC≡CH + HBr → RCH(Br)=CH<sub>2</sub>
And adds to the haloalkene to form a [[geminal]] dihaloalkane. (This type of addition follows [[Markovnikov's rule]]):
::RC(Br)=CH<sub>2</sub> + HBr → RC(Br<sub>2</sub>)–CH<sub>3</sub>
Also, HBr is used to open epoxides and lactones and in the synthesis of bromoacetals. Additionally, HBr catalyzes many organic reactions.<ref name="hercouet">Hercouet, A.;LeCorre, M. (1988) Triphenylphosphonium bromide: A convenient and quantitative source of gaseous hydrogen bromide. Synthesis, 157-158.</ref><ref name="greenwood">Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Butterworth-Heineman: Oxford, Great Britain; 1997; pp. 809-812.</ref><ref name="carlin">Carlin, William W. {{US Patent|4147601}}, April 3, 1979</ref><ref name="vollhart">Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function; 4th Ed.; W. H. Freeman and Company: New York, NY; 2003.</ref>
==Industrial preparation==
Unlike [[hydrogen chloride]] and [[hydrochloric acid]], which are major industrial chemicals, hydrogen bromide (along with hydrobromic acid) is produced on a much smaller scale. In the primary industrial preparation, [[hydrogen]] and [[bromine]] are combined at elevated temperatures (200-400 °C). The reaction is typically catalyzed by [[platinum]] or [[asbestos]].<ref name="greenwood"/><ref name="ruhoff"/>
==Laboratory synthesis==
HBr can synthesized by a variety of methods. A convenient laboratory synthesis entails the reaction between [[sulfuric acid]] and NaBr:<ref name="webele">[http://www.webelements.com/webelements/compounds/text/H/Br1H1-10035106.html WebElements: Hydrogen Bromide]</ref>
::NaBr<sub>(s)</sub> + H<sub>2</sub>SO<sub>4(aq)</sub> → NaHSO<sub>4(s)</sub> + HBr<sub>(g)</sub>
However, this synthetic process is ineffective as HBr formed will be oxidized to bromine gas by sulfuric acid:
::2HBr<sub>(g)</sub> + H<sub>2</sub>SO<sub>4(aq)</sub> → Br<sub>2(g)</sub> + SO<sub>2(g)</sub> + 2H<sub>2</sub>O<sub>(l)</sub>
Instead, non-oxidising acids like [[phosphoric acid]] or [[acetic acid]] can be used for the purpose.
Alternatively, it can be prepared by the bromination of tetraline (1,2,3,4-tetrahydronaphthalene):<ref name="webele"/>
::C<sub>10</sub>H<sub>12</sub> + 4Br<sub>2</sub> → C<sub>10</sub>H<sub>8</sub>Br<sub>4</sub> + 4HBr<sub>(g)</sub>
Reacting purified hydrogen gas and bromine (in the presence of a platinum catalyst):<ref name="webele"/>
::Br<sub>2</sub> + H<sub>2</sub> → 2HBr<sub>(g)</sub>
And reducing bromine with phosphorous acid:<ref name="greenwood" />
::Br<sub>2</sub> + H<sub>3</sub>PO<sub>3</sub> + H<sub>2</sub>O → H<sub>3</sub>PO<sub>4(s)</sub> + 2HBr<sub>(g)</sub>
Anhydrous hydrogen bromide can also be produced on a small scale (10 mmol-1 mol) through the [[thermolysis]] of [[triphenylphosphonium bromide]] in refluxing [[xylene]].<ref name="hercouet"/>
HBr prepared by the above methods can be contaminated with Br<sub>2</sub>, which can be removed by passing the gas through Cu turnings or through phenol.<ref name="ruhoff">Ruhoff, J. R.; Burnett, R. E.; Reid, E. E. "Hydrogen Bromide (Anhydrous)" Organic Syntheses, Vol. 15, p.35 (Coll. Vol. 2, p.338).</ref>
==References==
<references/>
[[Category:Inorganic compounds]]
[[Category:Hydrogen compounds]]
[[Category:Bromides]]
[[Category:Nonmetal halides]]
[[cs:Kyselina bromovodíková]]
[[de:Bromwasserstoff]]
[[hu:Hidrogén-bromid]]
[[nl:Waterstofbromide]]
[[ja:臭化水素]]
[[ru:Бромоводород]]
[[sv:Vätebromid]]
[[zh:溴化氢]]
[[fr:Bromure d'hydrogène]]