hydrogen bromide

Hydrogen bromide
IUPAC name	Hydrogen bromide
Other names	Bromane
Identifiers
CAS number	10035-10-6 Y
PubChem	260
EC number	233-113-0
UN number	1048
RTECS number	MW3850000
ChemSpider ID	255
Properties
Molecular formula	HBr
Molar mass	80.91 g/mol
Appearance	Colorless gas.
Density	3.307 g/L, gas.
Melting point	–86.80 °C (186.35 K)
Boiling point	–66.38 °C (206.77 K)
Solubility in water	193 g/100 ml (20 °C)
Acidity (pKa)	≈ –9
Refractive index (nD)	1.325
Structure
Molecular shape	Linear.
Dipole moment	0.82 D
Thermochemistry
Std enthalpy of formation ΔfHo298	-0.4486 kJ/g
Specific heat capacity, C	0.3507 J/g K
Hazards
MSDS	Hydrobromic acid Hydrogen bromide
EU Index	035-002-00-0
EU classification	Corrosive (C)
R-phrases	R35, R37
S-phrases	(S1/2), S7/9, S26, S45
NFPA 704	0 3 0
Related compounds
Other anions	Hydrogen fluoride, Hydrogen chloride, Hydrogen iodide
Related compounds	Hydrobromic acid
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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 bromooalkenes. 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.^[1][2][3][4]

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 200-400 °C. The reaction is typically catalyzed by platinum or asbestos.^[2][5]

Laboratory synthesis

HBr can synthesized by a variety of methods. A convenient laboratory synthesis entails the reaction between strong acids and NaBr:^[6] Sulfuric acid is ineffective because HBr formed will be oxidized to bromine gas:

2 HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O

Non-oxidising acids like phosphoric acid can be used for the purpose. Alternatively, it can be prepared by the bromination of tetraline (1,2,3,4-tetrahydronaphthalene):^[6]

C₁₀H₁₂ + 4 Br₂ → C₁₀H₈Br₄ + 4 HBr

Alternatively bromine can be reduced with phosphorous acid:^[2]

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.^[1]

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.^[5]

References

1. ^ ^{a b} Hercouet, A.;LeCorre, M. (1988) Triphenylphosphonium bromide: A convenient and quantitative source of gaseous hydrogen bromide. Synthesis, 157-158.
2. ^ ^{a b c} Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Butterworth-Heineman: Oxford, Great Britain; 1997; pp. 809-812.
3. ^ Carlin, William W. U.S. Patent 4,147,601, April 3, 1979
4. ^ Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function; 4th Ed.; W. H. Freeman and Company: New York, NY; 2003.
5. ^ ^{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).
6. ^ ^{a b} WebElements: Hydrogen Bromide