Distinguish bromine addition from radical substitution and follow the two-stage sulfuric-acid route from an alkene to an alcohol.
A non-polar reagent can become polarised
Br₂ has no permanent dipole. As it approaches an alkene, the electron-rich π bond repels electrons in Br₂, inducing Brδ+–Brδ−. The nearer δ+ bromine accepts an electron pair. Addition under suitable non-aqueous conditions places one Br on each original double-bond carbon. No UV initiation is needed.
For but-2-ene the dibromo product is 2,3-dibromobutane. The bromine atoms attach to the two carbons that were double-bonded, not both to one carbon.
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Bromine addition: school-level electron-pair model
Labels to include:
- induced Brδ+–Brδ−
- C=C pair to Brδ+
- Br–Br pair to departing Br
- positively charged intermediate
- Br⁻ attack
- adjacent C–Br bonds
In the simplified AQA-style representation, start an arrow at C=C and point to the nearer Brδ+. Send the Br–Br pair to the other Br, then show Br⁻ donating a lone pair to the positive carbon. The product has Br on each original alkene carbon. The more detailed chemical description uses a bridged bromonium intermediate; the simple representation is a teaching model and should not be used to infer the full stereochemistry.
Bromine water is an observation, not an identity certificate
Shake a small sample with bromine water at room temperature, avoiding UV conditions. For a simple alkene the orange/brown colour is lost. A simple alkane shows no rapid decolourisation under these conditions. Say “colourless”, not merely “clear”: a coloured solution can be transparent.
Aqueous bromine can give products involving water as well as bromide, so do not assume every bromine-water test produces only the isolated dibromo compound. The test observation is useful evidence for unsaturation in the stated set of candidates; other reactive substances can also decolourise bromine.
When a question gives quantitative bromine addition to C=C, one mole of Br₂ is consumed per mole of double bonds reacted. State the assumption that other bromine-consuming reactions are absent.
Concentrated sulfuric acid adds across C=C
At room temperature, an alkene can react with concentrated sulfuric acid to make an alkyl hydrogensulfate. For ethene the product is CH₃CH₂OSO₃H, ethyl hydrogensulfate. The organic carbon is bonded to oxygen, not directly to sulfur.
Show the acid as H–O–S(=O)₂–OH so the relevant H–O bond is explicit. An arrow from C=C goes to H; another from that H–O bond goes to O. This produces a carbocation and HSO₄⁻. A lone pair on the negatively charged oxygen of HSO₄⁻ then attacks the positive carbon.
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Sulfuric-acid addition mechanism
Labels to include:
- H–O bond in H₂SO₄
- C=C pair to H
- H–O pair to O
- carbocation
- O lone pair of HSO₄⁻
- C–O–S linkage
Keep the hydrogen sulfate structure and negative charge visible. The attacking oxygen makes the bond to carbon. A bare arrow from the text HSO₄⁻ without an identified oxygen lone pair does not explain where the bonding electrons originate.
Warm with water to obtain the alcohol
Hydrolysis of the alkyl hydrogensulfate gives an alcohol and regenerates sulfuric acid. Add this equation to the addition equation: the net change is hydration of the alkene. This two-stage route differs from industrial direct hydration with steam and a phosphoric-acid catalyst.
| Starting material and reagent | Conditions | Main change |
|---|---|---|
| Alkene + Br₂ | Room temperature, no UV required | Addition to C=C |
| Alkane + Cl₂ | UV irradiation | Free-radical substitution of H |
| Alkene + concentrated H₂SO₄, then water | Addition near room temperature, then warm hydrolysis | Alcohol through an alkyl hydrogensulfate |
| Ethene + steam | Industrial heat/pressure, H₃PO₄ catalyst | Direct hydration to ethanol |
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Name the product from bromine addition to pent-2-ene under non-aqueous addition conditions.Show answer
2,3-Dibromopentane, CH₃CHBrCHBrCH₂CH₃. Each original double-bond carbon gains one Br.
Q2. How is Br₂ able to behave as an electrophilic reagent despite having no permanent dipole?Show answer
The approaching alkene’s π electrons induce a dipole in Br₂. The nearer bromine becomes δ+ and accepts an electron pair.
Q3. A molecule has three reactive C=C bonds. How many moles of Br₂ are needed for complete addition to 0.00400 mol?Show answer
3 × 0.00400 = 0.0120 mol Br₂, assuming only those double bonds consume bromine.
Q4. What atom of HSO₄⁻ bonds to carbon in sulfuric-acid addition?Show answer
Oxygen, using an oxygen lone pair. The product contains a C–O–S linkage, not a new C–S bond.
Q5. Write the net reaction after ethyl hydrogensulfate formation and hydrolysis, and identify the acid’s overall role.Show answer
CH₂=CH₂ + H₂O → CH₃CH₂OH. H₂SO₄ is regenerated across the two steps, so its overall role is catalyst.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- Chemrevise: Alkenes — Coverage checklist, pp1–4; original teaching examples and practice.
- AQA 7405 specification — 3.3.4.1–3.3.4.3.
- AQA June 2022 AS Paper 2 mark scheme — Q07.6 p28: sulfuric-acid addition arrows.
- AQA June 2022 AS Paper 2 examiner report — Q07.6 p5: hydrogen sulfate charge.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.