Hydrogen bromide

Chembox new
Name = Hydrogen bromide
ImageFile = Hydrogen-bromide-2D-dimensions.png ImageName = Hydrogen bromide
ImageFile1 = Hydrogen-bromide-3D-vdW.png ImageName1 = Hydrogen bromide
PubChem = 260
IUPACName = Hydrogen Bromide
OtherNames =
Section1 = Chembox Identifiers
CASNo = 10035-10-6
ChemSpiderID = 255
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 D
Section7 = Chembox Hazards
ExternalMSDS = [http://www.jtbaker.com/msds/englishhtml/H3768.htm hydrobromic acid]
[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 HBr. 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⁺ and Br^– 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 H₂O 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⁺OH₂ + Br^– → RBr + H₂O HBr adds to alkenes to give bromoalkanes:::RCH=CH₂ + HBr → RCH(Br)–CH₃

HBr adds to alkynes to yield haloalkenes. The stereochemistry of this type of addition is usually "anti")::RC≡CH + HBr → RCH(Br)=CH₂

And adds to the haloalkene to form a geminal dihaloalkane. (This type of addition follows Markovnikov's rule):::RC(Br)=CH₂ + HBr → RC(Br₂)–CH₃Also, HBr is used to open epoxides and lactones and in the synthesis of bromoacetals. Additionally, HBr catalyzes many organic reactions.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. US Patent|4147601, 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.]

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.

Laboratory synthesis

HBr can synthesized by a variety of methods. A convenient laboratory synthesis entails the reaction between sulfuric acid and NaBr: [http://www.webelements.com/webelements/compounds/text/H/Br1H1-10035106.html WebElements: Hydrogen Bromide] ] ::NaBr_(s) + H₂SO_4(aq) → NaHSO_4(s) + HBr_(g)However, this synthetic process is ineffective as HBr formed will be oxidized to bromine gas by sulfuric acid:::2HBr_(g) + H₂SO_4(aq) → Br_2(g) + SO_2(g) + 2H₂O_(l)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):::C₁₀H₁₂ + 4Br₂ → C₁₀H₈Br₄ + 4HBr_(g)Reacting purified hydrogen gas and bromine (in the presence of a platinum catalyst):::Br₂ + H₂ → 2HBr_(g)And reducing bromine with phosphorous acid:::Br₂ + H₃PO₃ + H₂O → H₃PO_4(s) + 2HBr_(g)Anhydrous hydrogen bromide can also be produced on a small scale (10 mmol-1 mol) through the thermolysis of triphenylphosphonium bromide in refluxing xylene.

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.Ruhoff, J. R.; Burnett, R. E.; Reid, E. E. "Hydrogen Bromide (Anhydrous)" Organic Syntheses, Vol. 15, p.35 (Coll. Vol. 2, p.338).]

References