chemistry

Hydrogen bromide

Hydrogen bromide is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Hydrogen bromide rather than just read about it. In short: Hydrogen bromide is the inorganic compound with the formula HBr. It is a hydrogen halide consisting of hydrogen and bromine.

Key takeaways

Hydrogen bromide belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Hydrogen bromide to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Hydrogen bromide from memory before moving on to harder problems.

Reference excerpt

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.

Reactions

Organic chemistry Hydrogen bromide and hydrobromic acid are important reagents in the production of organobromine compounds. 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.

P(C6H5)3 + HBr → [HP(C6H5)3]+Br−

Inorganic chemistry Vanadium(III) bromide and molybdenum(IV) bromide were prepared by treatment of the higher chlorides with HBr. These reactions proceed by a combination halide exchange and redox reactions:

2 VCl4 + 8 HBr → 2 VBr3 + 8 HCl + Br2 Hydrogen bromide is used for the production of anhydrous ferrous bromide by oxidation of metallic iron:

Fe + 2 HBr → FeBr2 + H2 By contrast, use of hydrobromic acid gives the hexahydrate FeBr2·(H2O)6.

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 refluxing xylene. 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.

Safety HBr is highly corrosive and, if inhaled, can cause lung damage.

References

Illustrations

Worked examples

Example 1 — a first encounter with Hydrogen bromide

Start with the simplest possible case. Write down what Hydrogen bromide claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Hydrogen bromide before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Hydrogen bromide ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Hydrogen bromide

In research: Hydrogen bromide appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Hydrogen bromide in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Hydrogen bromide is common in secondary-school and first-year university syllabi. It links to neighbouring topics Bromides, Diatomic molecules, Hydrogen compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Hydrogen bromide outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Hydrogen bromide in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Hydrogen bromide means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Hydrogen bromide out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Hydrogen bromide in simple terms?

Hydrogen bromide is the inorganic compound with the formula HBr. It is a hydrogen halide consisting of hydrogen and bromine.

Why does Hydrogen bromide matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Hydrogen bromide?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Hydrogen bromide.