|Preferred IUPAC name
Hydrogen bromide
|Systematic IUPAC name
|Molar mass||80.91 g·mol−1|
|Density||3.6452 kg/m3 (0 °C, 1013 mbar)|
|Melting point||−86.9 °C (−124.4 °F; 186.2 K)|
|Boiling point||−66.8 °C (−88.2 °F; 206.3 K)|
|221 g/100 mL (0 °C)
204 g/100 mL (15 °C)
193 g/100 mL (20 °C)
130 g/100 mL (100 °C)
|Solubility||soluble in alcohol, organic solvents|
|Vapor pressure||2.308 MPa (at 21 °C)|
Refractive index (nD)
|350.7 mJ K−1 g−1|
|198.696-198.704 J K−1 mol−1|
Std enthalpy of
|-36.45--36.13 kJ mol−1|
|Safety data sheet||hazard.com
|GHS signal word||DANGER|
|P261, P280, P305+351+338, P310|
|S-phrases||(S1/2), S7/9, S26, S45|
|Lethal dose or concentration (LD, LC):|
LC50 (Median concentration)
|2858 ppm (rat, 1 hr)
814 ppm (mouse, 1 hr)
|US health exposure limits (NIOSH):|
|TWA 3 ppm (10 mg/m3)|
|TWA 3 ppm (10 mg/m3)|
IDLH (Immediate danger
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is: / ?)(|
Hydrogen bromide is the diatomic molecule with the formula HBr. HBr is a colorless gas that condenses to a colorless liquid. Hydrobromic acid is a solution of HBr in water. 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.
Solubility in water
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 47.6% HBr by weight form a constant-boiling azeotrope mixture that boils at 124.3 °C. Boiling less concentrated solutions releases H2O until the constant boiling mixture composition is reached.
Uses of HBr
Hydrogen bromide and hydrobromic acid are important reagents in the production of inorganic and organic bromine compounds. The free-radical addition of HBr to alkenes gives terminal alkyl bromides:
- RCH=CH2 + HBr → RCH2–CH2Br
- HBr + CH2Cl2 → HCl + CH2BrCl
- HBr + CH2BrCl → HCl + CH2Br2
Allyl bromide is prepared by treating allyl alcohol with HBr:
- CH2=CHCH2OH + HBr → CH2=CHCH2Br + H2O
Although not widely used industrially, HBr adds to alkenes to give bromoalkanes, an important family of organobromine compounds. Similarly, HBr adds to haloalkene to form a geminal dihaloalkane. (This type of addition follows Markovnikov's rule):
- RC(Br)=CH2 + HBr → RC(Br2)–CH3
- RC≡CH + HBr → RC(Br)=CH2
HBr has been proposed for use in a utility-scale flow-type battery.
HBr can be synthesized by a variety of methods. It may be prepared in the laboratory by distillation of a solution of sodium bromide or potassium bromide with phosphoric acid or diluted sulfuric acid:
- 2 KBr + H2SO4 → K2SO4 + 2HBr
Concentrated sulfuric acid is ineffective because HBr formed will be oxidized to bromine gas:
- 2 HBr + H2SO4 → Br2 + SO2 + 2H2O
- 2 Br2 + S + 2 H2O → 4 HBr + SO2
- C10H12 + 4 Br2 → C10H8Br4 + 4 HBr
Alternatively bromine can be reduced with phosphorous acid:
- Br2 + H3PO3 + H2O → H3PO4 + 2 HBr
Hydrogen bromide prepared by the above methods can be contaminated with Br2, which can be removed by passing the gas through a solution of phenol at room temperature in tetrachloromethane or other suitable solvent (producing 2,4,6-Tribromophenol and generating more HBr in the process) or through copper turnings at high temperature.
HBr is highly corrosive and irritating to inhalation.
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- Record in the GESTIS Substance Database of the IFA
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- Carlin, William W. U.S. Patent 4,147,601, April 3, 1979
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- M. Schmeisser "Chlorine, Bromine, Iodine" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 282.