Hydrogen bromide


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) + H2SO4(aq) → NaHSO4(s) + HBr(g)However, this synthetic process is ineffective as HBr formed will be oxidized to bromine gas by sulfuric acid:::2HBr(g) + H2SO4(aq) → Br2(g) + SO2(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 → H3PO4(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


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Look at other dictionaries:

  • hydrogen bromide — n a colorless irritating gas HBr that fumes in moist air and that yields hydrobromic acid when dissolved in water …   Medical dictionary

  • hydrogen bromide — Hydrobromic Hy dro*bro mic, a. [Hydro , 2 + bromic.] (Chem.) Composed of hydrogen and bromine; as, hydrobromic acid. [1913 Webster] {Hydrobromic acid} (Chem.), a colorless, pungent, corrosive gas, {HBr}, usually collected as a solution in water.… …   The Collaborative International Dictionary of English

  • hydrogen bromide — vandenilio bromidas statusas T sritis chemija formulė HBr atitikmenys: angl. hydrobromic acid; hydrogen bromide rus. бромоводород; водорода бромид …   Chemijos terminų aiškinamasis žodynas

  • hydrogen bromide — noun a colorless gas that yields hydrobromic acid in solution with water • Hypernyms: ↑bromide * * * noun : a colorless irritating gas HBr that fumes in moist air and yields hydrobromic acid when dissolved in water and that is formed as a by… …   Useful english dictionary

  • hydrogen bromide — noun Date: 1869 a colorless irritating gas HBr that fumes in moist air and yields hydrobromic acid when dissolved in water …   New Collegiate Dictionary

  • hydrogen bromide — a colorless gas, HBr, having a pungent odor: the anhydride of hydrobromic acid. [1895 1900] * * * …   Universalium

  • hydrogen bromide — noun A colourless gas having an irritating smell; it has the molecular formula HBr, and dissolves in water to form hydrobromic acid; it has many industrial applications including the production of barbiturates and synthetic hormones …   Wiktionary

  • hydrogen bromide — hy′drogen bro′mide n. chem. a colorless gas, HBr, having a pungent odor: the anhydride of hydrobromic acid • Etymology: 1895–1900 …   From formal English to slang

  • hydrogen bromide — /haɪdrədʒən ˈbroʊmaɪd/ (say huydruhjuhn brohmuyd) noun a colourless, poisonous gas, HBr, with a pungent smell …   Australian English dictionary

  • hydrogen — /huy dreuh jeuhn/, n. a colorless, odorless, flammable gas that combines chemically with oxygen to form water: the lightest of the known elements. Symbol: H; at. wt.: 1.00797; at. no.: 1; density: 0.0899 g/l at 0°C and 760 mm pressure. [1785 95;… …   Universalium


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