Hydrogen bromide
From Wikipedia, the free encyclopedia
| Hydrogen bromide | |
|---|---|
|
|
| General | |
| Systematic name | Hydrogen Bromide |
| Other names | |
| Molecular formula | HBr |
| Molar mass | 80.912 g/mol |
| Appearance | Colorless gas. |
| CAS number | [10035-10-6] [1] |
| Properties | |
| Density and phase | 3.307 g/L, gas. |
| Solubility in water | 193 g/100 ml (20 °C) |
| Melting point | –86.80°C (186.35 K) |
| Boiling point | –66.38°C (206.77 K) |
| Critical point | Tcrit = 90°C Pcrit = 8.5 MPa |
| Acidity (pKa) | ≈ –9 |
| Structure | |
| Molecular shape | Linear. |
| Dipole moment | 0.82 D |
| Hazards | |
| MSDS | hydrobromic acid hydrogen bromide |
| Main hazards | Toxic, corrosive. |
| NFPA 704 |
0
3
1
|
| Flash point | Non-flammable. |
| R/S statement | R: R23, R35, R37. S: S7, S9, S26, S36, S37, S39, S45. |
| RTECS number | MW3850000 |
| Thermodynamic data | |
| Standard enthalpy of formation |
ΔfH0 (gas) = –36.3 kJ/mol ΔfH0 (aq, ∞) = –121.0 kJ/mol |
| Heat of vaporization | ΔHvap = 217 J/g |
| Heat of fusion | ΔHfus = 29.8 J/g |
| Heat of solution | ΔHsol = –83.7 kJ/mol |
| Supplementary data page | |
| Structure and properties |
n, εr, etc. |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds | |
| Other anions | HF HCl HI |
| Other cations | ? |
| Related compounds | ? |
| Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Infobox disclaimer and references |
|
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.
Contents |
[edit] 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 solution that is more concentrated than this causes HBr to boil in excess until the constant boiling mixture is reached. Boiling solutions less concentrated causes H2O to boil in excess until the constant boiling mixture is reached.
[edit] 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)–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]
[edit] Laboratory synthesis of HBr
HBr can synthesized by a variety of methods. A convenient laboratory synthesis entails the reaction between sulfuric acid and NaBr:[5]
-
- NaBr(s) + H2SO4(aq) → NaHSO4(s) + HBr(g)
The bromination of tetraline (1,2,3,4 tetrahydronaphthalene):[5]
-
- C10H12 + 4Br2 → C10H8Br4 + 4HBr(g)
Reacting purified hydrogen gas and bromine (in the presence of a platinum catalyst):[5]
-
- Br2 + H2 → 2HBr(g)
And reducing bromine with phosphorous acid:2
-
- 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.[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.[6]
[edit] 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.[2][6]
[edit] References
- ^ a b Hercouet, A.;LeCorre, M. (1988) Triphenylphosphonium bromide: A convenient and quantitative source of gaseous hydrogen bromide. Synthesis, 157-158.
- ^ a b Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Butterworth-Heineman: Oxford, Great Britain; 1997; pp. 809-812.
- ^ Carlin, William W. U.S. 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.
- ^ a b c WebElements: Hydrogen Bromide
- ^ 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).
