AQA A-Level Chemistry: Alkenes (7405) – Revision Notes

These notes condense section 3.3.4 Alkenes of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: 3.3.4.1 Structure, bonding and reactivity, 3.3.4.2 Addition reactions of alkenes and 3.3.4.3 Addition polymers. All of it is AS content, so nothing here is A-level only. It is examined on AS Paper 2 and A-level Paper 2, and A-level Paper 3 can test any content.

For full explanations and worked examples, read the alkenes study guide first. Then test yourself with the alkenes practice questions. The course hub is AQA A-Level Chemistry, and the printable checklist lists every specification point. The free 10-minute diagnostics help you find weak spots. For intermolecular forces, see bonding.

Key definitions

Term	Meaning
Unsaturated	Contains at least one C=C double bond
Alkene	Unsaturated hydrocarbon; non-cyclic with one C=C: CₙH₂ₙ
Electrophile	Electron pair acceptor (e.g. H⁺, Hδ+ in HBr, Brδ+ in Br₂)
Electrophilic addition	An electrophile attacks the C=C; two species add across it; product is saturated
Carbocation	Ion with a positively charged carbon atom
Unsymmetrical alkene	The two C=C carbons carry different groups (e.g. propene)
Major / minor product	Product formed via the more / less stable carbocation
Addition polymer	Long saturated chain formed when many alkene monomers join; no other product
Repeating unit	The two-carbon unit from one monomer, drawn with extension bonds and n
Plasticiser	Small molecules that get between polymer chains and make the polymer flexible

Why alkenes react with electrophiles (3.3.4.1)

• A C=C bond has two shared pairs between the same two carbons.
• So it is a centre of high electron density.
• Electrophiles (electron-deficient) are attracted to it and accept a pair of electrons from it.

Electrophilic addition: method in steps (3.3.4.2)

Method: outline any electrophilic addition mechanism

1. Show the dipole on the electrophile (Hδ+–Brδ−; induced Brδ+–Brδ−; H in H₂SO₄).
2. Arrow 1: from the middle of the C=C bond to the δ+ atom.
3. Arrow 2: from the bond in the electrophile to the δ− atom (heterolytic fission).
4. Draw the carbocation with + on the correct carbon, and the negative ion.
5. Arrow 3: from a lone pair on the negative ion to the C⁺.
6. Draw the product with the new group on the former C⁺.

Reagent	Conditions	Electrophile	Product from ethene
HBr	room temperature	Hδ+	bromoethane, CH₃CH₂Br
Br₂	room temperature	induced Brδ+	1,2-dibromoethane, CH₂BrCH₂Br
H₂SO₄	cold, concentrated	H of H₂SO₄	ethyl hydrogensulfate, CH₃CH₂OSO₂OH

With H₂SO₄, the hydrogensulfate ion bonds to the carbon through its negative O atom. Hydration of alkenes to make alcohols is in the alcohols study guide.

Small reminder (skeleton for HBr with but-2-ene):

CH₃CH=CHCH₃ + Hδ+–Brδ−  →  CH₃CH₂C⁺HCH₃ + Br⁻   (secondary carbocation)
CH₃CH₂C⁺HCH₃ + :Br⁻     →  CH₃CH₂CHBrCH₃        (2-bromobutane)

Three arrows in total: C=C → H, H–Br bond → Br, lone pair on Br⁻ → C⁺.

Bromine test for unsaturation

• Shake with bromine water at room temperature.
• Alkene: orange → colourless.
• Alkane: stays orange.
• The alkene test is part of Required practical 6, which the specification lists under 3.3.6.1.

Major and minor products

Carbocation	Alkyl groups on C⁺	Stability
primary	1	lowest
secondary	2	middle
tertiary	3	highest

Method: predict the major product

1. Check the alkene is unsymmetrical and the reagent is HBr or H₂SO₄ (not Br₂).
2. Draw both carbocations (H adds to one C=C carbon, + goes on the other).
3. Classify each as primary, secondary or tertiary.
4. More alkyl groups → positive charge spread more (electron-releasing) → more stable → forms more readily.
5. The product from the more stable carbocation is the major product.

Small reminder. 2-methylpropene, (CH₃)₂C=CH₂, with HBr:

• tertiary carbocation (CH₃)₃C⁺ → 2-bromo-2-methylpropane (major)
• primary carbocation (CH₃)₂CHCH₂⁺ → 1-bromo-2-methylpropane (minor)

Addition polymers (3.3.4.3)

Method: monomer → repeating unit

1. Draw the monomer with its four groups around C=C.
2. Change C=C to C–C.
3. Keep each group on the same carbon.
4. Add extension bonds from both carbons, brackets and n.

Method: section of chain → repeating unit → monomer

1. Find the two-carbon pattern that repeats.
2. That pattern (with extension bonds) is the repeating unit.
3. Put a C=C between those two carbons to get the monomer.

Naming: poly(monomer name).

Monomer	Formula	Polymer
ethene	CH₂=CH₂	poly(ethene)
propene	CH₂=CHCH₃	poly(propene)
chloroethene	CH₂=CHCl	poly(chloroethene), PVC
tetrafluoroethene	CF₂=CF₂	poly(tetrafluoroethene)
phenylethene	CH₂=CHC₆H₅	poly(phenylethene)

Small reminder. Average Mr of a poly(tetrafluoroethene) sample = 5.0 × 10⁵. Mr(C₂F₄) = 2(12.0) + 4(19.0) = 100.0. Units per chain = 5.0 × 10⁵ / 100.0 = 5000.

Properties of polyalkenes

• Unreactive: the chain is saturated, with strong, non-polar C–C and C–H bonds; no C=C for electrophiles to attack.
• Intermolecular forces: van der Waals forces between chains. Very long chains have many electrons and a large contact area, so the total attraction is large.
• PVC also has permanent dipole–dipole forces, from the polar C–Cl bond.

PVC and plasticisers

Form	Property	Typical uses
Unplasticised PVC	rigid	window frames, drainpipes, guttering
Plasticised PVC	flexible	electrical cable insulation, flooring

A plasticiser’s molecules sit between the chains, push them apart and weaken the intermolecular forces, so the chains slide more easily.

Development over time: understanding of polymers has grown. For example, poly(ethene) was first made at very high pressure (branched chains); catalysts later allowed lower-pressure manufacture of less-branched, denser poly(ethene). Plasticisers show how one polymer’s properties can be changed.

Practical opportunity: the specification suggests making poly(phenylethene) from phenylethene (AT k, PS 1.2). It is not a required practical.

Must-know distinctions

• Addition vs substitution: alkenes add (two species in, nothing out); alkanes substitute.
• HBr vs Br₂ with propene: HBr gives two products (major and minor); Br₂ gives one product, 1,2-dibromopropane.
• Symmetrical vs unsymmetrical: but-2-ene gives only 2-bromobutane with HBr; but-1-ene gives two products.
• Monomer vs repeating unit: the monomer has C=C; the repeating unit has C–C and extension bonds.
• Unreactive vs strong forces: unreactivity is about the strong C–C and C–H bonds within chains; the solid state is about van der Waals forces between chains.

Quick self-test

1. State what is meant by an electrophile.
2. Why is a C=C bond attacked by electrophiles?
3. Name the product of ethene and bromine.
4. State the colour change when cyclohexene is shaken with bromine water.
5. Identify the electrophile when ethene reacts with H₂SO₄.
6. Classify the carbocation CH₃CH₂C⁺(CH₃)₂.
7. Name the major product of but-1-ene with HBr.
8. Why does but-2-ene give only one product with HBr?
9. Give the name of the polymer made from CH₂=CHCH₃.
10. A section of chain reads –CHCl–CHCl–CHCl–CHCl–. Give the monomer.
11. A poly(phenylethene) sample has average Mr 3.12 × 10⁵. Use Mr(C₈H₈) = 104.0 to find the average number of monomer units.
12. Explain how a plasticiser makes PVC flexible.

Answers

1. An electron pair acceptor.
2. It is a centre of high electron density (two shared pairs of electrons).
3. 1,2-dibromoethane.
4. Orange to colourless.
5. A hydrogen (Hδ+) of H₂SO₄.
6. Tertiary (three alkyl groups on C⁺).
7. 2-bromobutane (via the secondary carbocation).
8. It is symmetrical, so both possible carbocations are identical and give the same product.
9. Poly(propene).
10. CHCl=CHCl, 1,2-dichloroethene.
11. 3.12 × 10⁵ / 104.0 = 3000.
12. Its molecules get between the chains, push them apart and weaken the intermolecular forces, so the chains slide past each other.

Where marks are usually lost

• First curly arrow drawn from a carbon atom, not from the C=C bond.
• No dipole on H–Br, or no induced dipole on Br₂.
• Arrow 3 starting from the minus sign instead of a lone pair.
• Carbocation + placed on the wrong carbon for the product named.
• Hydrogensulfate attached through S or through the OH group.
• “Tertiary carbocations are more reactive” instead of “more stable”.
• Stating major and minor products for Br₂ addition.
• Repeating unit drawn with a C=C, or without extension bonds and n.
• Substituent moved to the wrong carbon when drawing a monomer from a chain.
• Saying polyalkenes are unreactive because their intermolecular forces are strong.

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.3.4 Alkenes (3.3.4.1 Structure, bonding and reactivity, 3.3.4.2 Addition reactions of alkenes and 3.3.4.3 Addition polymers).