Hydrogen bromide is the inorganic compound with the formula . 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. Boiling less concentrated solutions releases H2O until the constant-boiling mixture composition is reached. Hydrogen bromide, and its aqueous solution, are commonly used reagents in the preparation of bromide compounds. Reactions Organic chemistry Hydrogen bromide and hydrobromic acid are important reagents in the production of organobromine compounds. In a free-radical reaction, HBr adds to alkenes: 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: 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: 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. Inorganic chemistry Vanadium(III) bromide and molybdenum(IV) bromide were prepared by treatment of the higher chlorides with HBr. These reactions proceed via redox reactions: Industrial preparation 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. Laboratory synthesis HBr can be prepared by distillation of a solution of sodium bromide or potassium bromide with phosphoric acid or sulfuric acid: 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: 2 Br2 + S + 2 H2O → 4 HBr + SO2 bromination of tetralin: C10H12 + 4 Br2 → C10H8Br4 + 4 HBr reduction of bromine with phosphorous acid: Br2 + H3PO3 + H2O → H3PO4 + 2 HBr Anhydrous hydrogen bromide can also be produced on a small scale by thermolysis of triphenylphosphonium bromide in