Hydrogen bromide

Lore & Background

Hydrogen bromide, and its aqueous solution, hydrobromic acid, are commonly used reagents in the preparation of bromide compounds. In organic chemistry, HBr adds to alkenes in an electrophilic addition reaction, and related free radical additions to allyl chloride and styrene give 1-bromo-3-chloropropane and phenylethylbromide, respectively. It reacts with dichloromethane to give bromochloromethane and dibromomethane. Allyl bromide is prepared by treating allyl alcohol with HBr. HBr adds to alkynes to yield bromoalkenes, and also adds to epoxides and lactones, resulting in ring-opening. With triphenylphosphine, HBr gives triphenylphosphonium bromide. In inorganic chemistry, vanadium(III) bromide and molybdenum(IV) bromide are prepared by treatment of the higher chlorides with HBr. Hydrogen bromide is used for the production of anhydrous ferrous bromide by oxidation of metallic iron.

Physical Character and Aqueous Behavior

Hydrogen bromide exists as a colorless gas with the molecular formula HBr, classifying it as a hydrogen halide formed from the union of hydrogen and bromine atoms. Its most notable practical characteristic is its solubility in water, where it dissolves to yield hydrobromic acid. At room temperature, the aqueous solution reaches saturation at 68.85 percent HBr by weight. A particularly interesting thermodynamic feature emerges at a lower concentration: a solution containing 47.6 percent HBr by mass forms a constant-boiling azeotropic mixture that distills at 124.3 degrees Celsius. This azeotrope behavior has direct practical consequences for purification. When one attempts to boil a less concentrated hydrobromic acid solution, water preferentially evaporates until the liquid composition converges on that 47.6 percent threshold. Beyond its role as a reagent in the preparation of various bromide compounds, this azeotropic behavior makes hydrobromic acid a distinctive member of the hydrohalic acid family, setting it apart from simpler volatile acids that can be concentrated indefinitely by distillation.

Reactivity in Organic Synthesis

In organic laboratories, hydrogen bromide and its aqueous counterpart serve as versatile reagents for constructing organobromine compounds. The most fundamental transformation is electrophilic addition across carbon-carbon double bonds: an alkene such as RCH=CH2 accepts HBr to yield an alkyl bromide, which in turn functions as a useful alkylating agent and a precursor to fatty amine derivatives. Free-radical pathways extend this chemistry to substrates like allyl chloride and styrene, producing 1-bromo-3-chloropropane and phenylethylbromide respectively. HBr also participates in sequential halide-exchange metathesis with dichloromethane, first generating bromochloromethane and then dibromomethane, a process driven by the thermodynamic preference to consume the stronger acid while liberating the weaker HCl. Converting allyl alcohol into allyl bromide represents another classic substitution. Addition across triple bonds furnishes bromoalkenes, typically with anti stereochemistry, while epoxides and lactones undergo ring-opening. Finally, reaction with triphenylphosphine yields triphenylphosphonium bromide, a convenient solid reservoir of HBr for downstream use.

Inorganic Applications and Routes of Preparation

Beyond organic synthesis, hydrogen bromide plays a significant role in inorganic chemistry. Treatment of higher vanadium and molybdenum chlorides with HBr produces vanadium(III) bromide and molybdenum(IV) bromide through coupled halide-exchange and redox processes. The compound also enables preparation of anhydrous ferrous bromide by direct oxidation of metallic iron, whereas the aqueous hydrobromic acid route yields the hexahydrate instead. On an industrial scale, HBr is manufactured by combining elemental hydrogen with bromine at temperatures spanning 200 to 400 degrees Celsius, with platinum or asbestos serving as catalysts. In the laboratory, smaller quantities are obtained by distilling sodium or potassium bromide solutions with phosphoric or sulfuric acid. However, concentrated sulfuric acid proves less effective because it oxidizes the liberated HBr into bromine and sulfur dioxide. Alternative laboratory routes include reacting bromine with water and sulfur, brominating tetralin, or reducing bromine with phosphorous acid. Anhydrous HBr on a small scale can also be generated by thermolyzing triphenylphosphonium bromide in refluxing xylene.

Handling Hazards and Gas Purification

Working with hydrogen bromide demands considerable caution. The gas is highly corrosive, and inhalation exposure can inflict serious damage to lung tissue, making proper ventilation and protective equipment essential in any setting where HBr is generated or handled. A recurring practical challenge is contamination of the product gas with elemental bromine, a byproduct of several synthesis routes. Two established methods exist for removing this impurity. The first involves passing the gas through a phenol solution dissolved in tetrachloromethane or a comparable solvent at room temperature; the bromine reacts with phenol to form 2,4,6-tribromophenol, and the process simultaneously generates additional HBr, so no net loss of the desired product occurs. The second approach routes the gas over copper turnings or copper gauze maintained at elevated temperature, where the copper scavenges the bromine. Both techniques are straightforward but critical for obtaining a clean, bromine-free stream of hydrogen bromide suitable for sensitive downstream reactions.

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