Hydrobromic Acid
Properties
| State | Liquid (aqueous solution) at room temperature |
| Color | Colorless to pale yellow |
| Solubility | Miscible with water |
| Melting Point | -11°C (47% solution) |
| Boiling Point | 122°C (47% solution) |
About Hydrobromic Acid
Hydrobromic acid (HBr in water, 80.912 g/mol) is the strongest of the practical halogen acids — pKa around -9, fully dissociated in water, and notably more acidic than HCl because Br's larger orbital makes the H-Br bond weaker (only 366 kJ/mol versus 432 for H-Cl). The concentrated commercial product is 48% HBr by mass, which is the constant-boiling azeotrope at 124 °C. In organic chemistry, HBr is the standard reagent for converting alcohols to alkyl bromides via SN2 (for primary) or SN1/E1 (for tertiary) — the route every sophomore learns when making 1-bromobutane from butanol with HBr/H2SO4. It's also the textbook acid for Markovnikov addition across alkenes (HBr-H goes to the carbon with more hydrogens, Br to the more substituted carbon, via the more stable carbocation intermediate). The anti-Markovnikov version requires radical conditions (HBr + peroxide), which is the only HX that does this — HCl bonds are too strong to fragment, and HI bonds are too weak to add efficiently. Industrially, HBr is the precursor for inorganic bromides (NaBr, KBr), pharmaceutical intermediates (think dextromethorphan hydrobromide), and the alkyl bromides used as alkylating agents. Concentrated HBr fumes badly in moist air and turns yellow-orange from trace Br2 formed by oxidation, which is why old bottles look amber.
Where you'll encounter it
If you've ever made an alkyl bromide via the alcohol-to-halide reaction, taken a cough suppressant containing dextromethorphan HBr, or worked with KBr disks in IR spectroscopy (the KBr was made by neutralizing HBr), you've intersected with this acid. Every bottle of Robitussin DM and every generic dextromethorphan cough syrup on a pharmacy shelf contains the active ingredient as dextromethorphan hydrobromide — the HBr salt is what gives the molecule water solubility and shelf stability. Older IR labs that still run KBr-pellet sample preparation grind unknowns with KBr powder synthesized originally from HBr neutralization. Organic chemists reach for the 48% azeotrope when converting an alcohol to its bromide for SN2 displacement chemistry — a few hours under reflux with H2SO4 as a dehydrating sidekick gives clean primary alkyl bromide for the next step in a synthesis.
Common Uses
- Conversion of primary and secondary alcohols to alkyl bromides via Lucas-type chemistry
- Markovnikov HBr addition to alkenes (and anti-Markovnikov with peroxide initiators)
- Synthesis of NaBr, KBr, CaBr2, and ZnBr2 by neutralization with the corresponding base
- Pharmaceutical salt formation (e.g., dextromethorphan HBr, scopolamine HBr)
- Etchant for III-V semiconductors and certain metal oxides
- Catalyst in some Friedel-Crafts and cyclization reactions
- Component of brine completion fluids in oil and gas drilling
Safety Information
Highly corrosive — concentrated HBr causes severe burns to skin, eyes, and respiratory tract. Releases HBr vapor that fumes in moist air and is intensely irritating to mucous membranes. OSHA PEL is 3 ppm (ceiling); ACGIH TLV is 2 ppm (ceiling). GHS: H314 (causes severe burns and eye damage), H335 (respiratory irritation), H400 (very toxic to aquatic life). Always handle in a fume hood. Wear nitrile gloves, splash goggles, and a face shield for any pour or transfer above 100 mL. Keep calcium gluconate and water nearby. Will attack most metals (releases H2) — store and pipette only in glass, PTFE, or PP.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.