Hydrogen bromide

H 1
Br 1

Ball-and-stick Stick Space-filling

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2 atoms, 1 single bond.

IUPAC name: bromane
Molar mass: 80.9120(32) g/mol
Atoms: 2
Polarity: polar
SMILES: Br
InChIKey: CPELXLSAUQHCOX-UHFFFAOYSA-N
PubChem CID: 260

Made of

H Br

Structural formula of hydrogen bromide. Every atom in the molecule is drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Hydrogen bromide is a gas at room temperature, with a sharp, irritating odour, and it is stored and shipped in cylinders compressed into a liquid. Its molecule is a single bromine atom bonded to a single hydrogen atom, giving a molar mass of 80.91 g/mol and the IUPAC name bromane. Its boiling point is −66.38 °C on the Hazardous Substances Data Bank’s figure, and the colourless gas is heavier than air.

Hydrobromic acid is a solution of the gas in water, and hydrogen bromide is often used in that form. The acid, made by dissolving the gas, is sold as 48 and 62 per cent solutions. The acid itself is colourless or faintly yellow and slowly darkens when left in air and light. The solution is a strong acid, and even the dry, water-free gas counts as one in the hazard database CAMEO Chemicals.

Structure and bonding

Hydrogen bromide is one hydrogen atom and one bromine atom, joined by one short single bond. In Space-filling, more than half of hydrogen’s small white sphere sinks into bromine’s dark red one, leaving a white dome on a much larger ball.

With bromine at 2.96 on the Pauling scale of electronegativity, an atom’s pull on the electrons it shares, and hydrogen at 2.2, the bond is polar. Chlorine pulls harder, at 3.16, yet hydrogen bromide boils higher than hydrogen chloride: −66.38 °C against −85.05 °C on the Hazardous Substances Data Bank’s figures.

Bromine is the larger atom, and on Chem1’s account a larger atom holds its outer electrons more loosely, so its electron cloud is more easily distorted, or polarisable. Dispersion forces, attractions between fleeting dipoles that shifting electrons induce in neighbouring molecules, depend on that polarisability (how they work).

The model shows the neutral molecule of the gas. In water it reacts virtually completely, as a strong acid does, and hands its protons to water molecules. Bromide ions, Br⁻, and hydronium ions, H₃O⁺, are left, and no single drawn molecule can show them.

Where it turns up

Industry makes hydrogen bromide by reacting hydrogen with bromine directly as gases. Another major supply is a by-product: when bromine is attached to certain organic compounds, hydrogen bromide comes off as well, less pure than the deliberately made gas. The gas sold in cylinders is commonly 99.8 per cent pure. According to the US Geological Survey, the by-product can be recycled with fresh bromine brines into bromine production (where the element comes from).

Beyond the chemical plant, workers meet it in metal extraction and refining, where it helps dissolve ores, and in photographic processing. Holographers use dilute hydrobromic acid as a colour bleach. Under the US Environmental Protection Agency (EPA) Chemical Data Reporting, it appears in plastics, electrical equipment and petrochemical manufacturing.

Why it matters

In industry hydrogen bromide is a way of carrying bromine into other compounds. Reacted with metal hydroxides, oxides or carbonates, hydrobromic acid gives inorganic bromides. Alcohols and alkenes, hydrocarbons with a carbon–carbon double bond, turn into organic bromides with it. The acid also cleaves ethers, compounds with an oxygen atom between two carbon groups, whether their chains are open or closed into rings.

It speeds up several kinds of organic reaction as a catalyst, oxidations among them. In analytical chemistry the acid dissolves ore minerals, and the gas is a pharmaceutical intermediate, a substance used up in the steps that make a medicine. Under the same EPA reporting it also serves in etching and plating and as a processing aid in petroleum production. The aggregated US volume of hydrobromic acid stood between 100 million and 500 million pounds in 2023 under that reporting, the band it held in each year from 2020.

Properties

Properties of hydrogen bromide Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 260.
Property	Value	Unit	Source
Molecular formula (Hill order)	BrH
Molar mass	80.9120(32) g/mol	g/mol	IUPAC/CIAAW 2024
Hydrogen atoms	1
Bromine atoms	1
Bonds	1
PubChem molecular weight	80.91 g/mol	g/mol	PubChem

Safety

Hydrogen bromide gas and hydrobromic acid are corrosive to the eyes, the skin and the respiratory tract, and the gas is toxic if inhaled. Its hazard labels carry the signal word Danger, and the liquefied gas can cause frostbite as it evaporates. Every hazard statement for hydrogen bromide is collected in its PubChem Laboratory Chemical Safety Summary. Workplace exposure limits are on the NIOSH Pocket Guide card from the US National Institute for Occupational Safety and Health.

Questions

How long is the H–Br bond in the 3D model?

In PubChem's computed structure the hydrogen and bromine nuclei sit 1.45 Å apart, against 1.31 Å for hydrogen and chlorine in the computed model of hydrogen chloride. Bromine brings 35 electrons in four shells, 2, 8, 18 and 7, where chlorine has 17 in three, and in Space-filling the bromine sphere is the larger of the two. These are computed distances, and the models draw only atoms and bonds.

How soluble is hydrogen bromide in water?

Very, for a gas. Gases usually dissolve in liquids only to a modest degree, but at 0 °C a single volume of water takes up 600 volumes of hydrogen bromide, according to the Hazardous Substances Data Bank (HSDB). Dissolving it releases a large amount of heat, and the solution formed is hydrobromic acid. In moist air the gas fumes, giving off clouds that contain the acid.

What is constant-boiling hydrobromic acid?

It is the concentration of hydrobromic acid that distils at one steady temperature. The Hazardous Substances Data Bank gives that temperature as 124.3 °C at atmospheric pressure, and the mixture contains 47.63 per cent hydrogen bromide by weight. That composition lies close to the 48 per cent solution sold commercially. Hydrogen bromide on its own boils at −66.38 °C, more than 190 degrees lower.

Is hydrogen bromide gas dangerous?

Yes. It is corrosive and toxic if inhaled, and breathing it may cause lung oedema, a build-up of fluid in the lungs. The gas is heavier than air, and vapour from the liquefied gas spreads along the ground at first. In short-term inhalation studies it proved more toxic than hydrogen chloride to rats and considerably more so to mice. A Japanese classification adds damage to the airways and teeth from repeated exposure.