Hydrogen bromide

Hydrogen bromide

Names IUPAC name Preferred IUPAC name Identifiers 3587158 ChEBI ChEMBL ChemSpider ECHA InfoCard 100.030.090 EC Number KEGG MeSH Hydrobromic+Acid RTECS number UNII UN number 1048 Properties HBr Molar mass 80.91 g/mol Appearance Colorless gas Odor Acrid Density 3.307 g/L (25 °C)[2] 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) Acidity (pKa) −8.8 (±0.8);[3] ~−9[4] Basicity (pKb) ~23 Conjugate acid Bromonium Conjugate base Bromide 1.325[citation needed] Structure Linear 820 mD Thermochemistry 350.7 mJ/(K·g) 198.696-198.704 J/(K·mol)[5] −36.45...−36.13 kJ/mol[5] Hazards Occupational safety and health (OHS/OSH): Highly corrosive GHS labelling: Danger H314, H335 P261, P280, P305+P351+P338, P310 NFPA 704 (fire diamond) Lethal dose or concentration (LD, LC): 2858 ppm (rat, 1 h)814 ppm (mouse, 1 h)[7] NIOSH (US health exposure limits): TWA 3 ppm (10 mg/m3)[6] TWA 3 ppm (10 mg/m3)[6] 30 ppm[6] Safety data sheet (SDS) hazard.com

physchem.ox.ac.uk

Related compounds Hydrogen fluorideHydrogen chlorideHydrogen iodideHydrogen astatide

Hydrogen bromide is the inorganic compound with the formula HBr. It is a hydrogen halide consisting of hydrogen and bromine. A colorless gas, it dissolves in water, forming hydrobromic acid, which is saturated at 68.85% HBr by weight at room temperature. Aqueous solutions that are 47.6% HBr by mass form a constant-boiling azeotrope mixture that boils at 124.3 °C (255.7 °F). Boiling less concentrated solutions releases H2O until the constant-boiling mixture composition is reached.

Hydrogen bromide, and its aqueous solution, hydrobromic acid, are commonly used reagents in the preparation of bromide compounds.

Hydrogen bromide and hydrobromic acid are important reagents in the production of organobromine compounds.[8][9][10] In an electrophilic addition reaction, HBr adds to alkenes:

RCH=CH2 + HBr → R−CHBr−CH3

The resulting alkyl bromides are useful alkylating agents, e.g., as precursors to fatty amine derivatives. Related free radical additions to allyl chloride and styrene give 1-bromo-3-chloropropane and phenylethylbromide, respectively.

Hydrogen bromide reacts with dichloromethane to give bromochloromethane and dibromomethane, sequentially:

HBr + CH2Cl2 → HCl + CH2BrCl HBr + CH2BrCl → HCl + CH2Br2

These metathesis reactions illustrate the consumption of the stronger acid (HBr) and release of the weaker acid (HCl).

Allyl bromide is prepared by treating allyl alcohol with HBr:

CH2=CHCH2OH + HBr → CH2=CHCH2Br + H2O

HBr adds to alkynes to yield bromoalkenes. The stereochemistry of this type of addition is usually anti:

RC≡CH + HBr → RC(Br)=CH2

Also, HBr adds epoxides and lactones, resulting in ring-opening.

With triphenylphosphine, HBr gives triphenylphosphonium bromide, a solid "source" of HBr.[11]

P(C6H5)3 + HBr → [HP(C6H5)3]+Br−

Vanadium(III) bromide and molybdenum(IV) bromide were prepared by treatment of the higher chlorides with HBr. These reactions proceed via redox reactions:[12]

2 VCl4 + 8 HBr → 2 VBr3 + 8 HCl + Br2

Hydrogen bromide (along with hydrobromic acid) is produced by combining hydrogen and bromine at temperatures between 200 and 400 °C. The reaction is typically catalyzed by platinum or asbestos.[9][13]

HBr can be prepared by distillation of a solution of sodium bromide or potassium bromide with phosphoric acid or sulfuric acid:[14]

KBr + H2SO4 → KHSO4 + HBr

Concentrated sulfuric acid is less effective because it oxidizes HBr to bromine:

2 HBr + H2SO4 → Br2 + SO2 + 2 H2O

The acid may be prepared by:

• reaction of bromine with water and sulfur:[14] 2 Br2 + S + 2 H2O → 4 HBr + SO2
• bromination of tetralin:[14] C10H12 + 4 Br2 → C10H8Br4 + 4 HBr
• reduction of bromine with phosphorous acid:[9] 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.[11]

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 or copper gauze at high temperature.[13]

HBr is highly corrosive and, if inhaled, can cause lung damage.[15]