OxfordAQA A-Level Chemistry: Alkenes (9620) – Revision Notes

A condensed version of section 3.3.4 Alkenes in the OxfordAQA International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards, split into 3.3.4.1 to 3.3.4.4: bonding and reactivity, addition reactions, addition polymers and epoxyethane. Every point is International AS. Alkenes are examined in Unit 2 (Organic 1 and Physical 1) and may resurface in synoptic Unit 5.

The alkenes study guide has the full explanations and worked examples, so start there. When these notes feel secure, move on to the alkenes practice questions. Every other topic appears on the OxfordAQA A-Level Chemistry hub and the printable checklist, while the free 10-minute diagnostics point you at weak areas. For the forces between chains, see shapes, polarity and intermolecular forces.

Key terms

Term	Meaning
Unsaturated	Has a C=C double bond, so more atoms can add on
Electrophile	Electron pair acceptor; positive or carries a δ+ atom
Electrophilic addition	Electrophile attacks C=C; two groups add; product saturated
Carbocation	Ion with a positively charged carbon
Induced dipole	Temporary δ+/δ− in Br₂ caused by the nearby C=C electrons
Repeating unit	Two main-chain carbons from one monomer, single bond, extension bonds, n
Plasticiser	Small molecules between polymer chains that make the polymer flexible
Surfactant (non-ionic)	Molecule with a non-polar tail and an uncharged polar head

3.3.4.1 Bonding and reactivity

• Alkene C=C = four shared electrons between two carbons → centre of high electron density.
• Electrophiles are attracted and accept a pair of electrons from it.
• Alkanes have only single bonds and no such region, so they do not react this way.

3.3.4.2 Addition reactions

Reagent	Conditions	Electrophile	Group that bonds to C⁺
HBr	room temperature	Hδ+	Br⁻
Br₂	room temperature	induced Brδ+	Br⁻
H₂SO₄	cold, concentrated	H of H₂SO₄	⁻OSO₂OH (via the O⁻)

Method: any electrophilic addition mechanism

1. Mark the δ+ and δ− (permanent in HBr, induced in Br₂).
2. Arrow from the middle of C=C to the δ+ atom.
3. Arrow from the bond in the reagent to the δ− atom.
4. Draw the carbocation (+ on the correct carbon) and the anion.
5. Arrow from a lone pair on the anion to C⁺.

Quick reminder (oct-1-ene + HBr):

CH₃(CH₂)₅CH=CH₂ + Hδ+–Brδ− → CH₃(CH₂)₅C⁺HCH₃ + Br⁻   (secondary, preferred)
CH₃(CH₂)₅C⁺HCH₃ + :Br⁻     → CH₃(CH₂)₅CHBrCH₃        (2-bromooctane, major)

The minor product, 1-bromooctane, comes from the primary carbocation CH₃(CH₂)₅CH₂CH₂⁺.

Bromine water test: shake with the sample. C=C present → orange to colourless. Saturated → no change, still orange. Required practical 5 (listed under 3.3.6.1) includes this test.

Method: major and minor products

1. Only for an unsymmetrical alkene with HBr or H₂SO₄.
2. Draw both carbocations.
3. Count alkyl groups on each C⁺: primary < secondary < tertiary in stability.
4. Alkyl groups release electrons and spread the charge.
5. More stable carbocation → forms more readily → major product.

3.3.4.3 Addition polymers

Method: monomer → repeating unit C=C becomes C–C; keep every group on its own carbon; add extension bonds, brackets and n.

Method: chain → monomer Find the two-carbon pattern that repeats; put C=C back between those two carbons.

Naming: poly(monomer name), e.g. methyl propenoate, CH₂=CHCOOCH₃ → poly(methyl propenoate).

Quick reminder: chain –CH₂–CF₂–CH₂–CF₂– → repeating unit –CH₂–CF₂– → monomer CH₂=CF₂, 1,1-difluoroethene.

Quick reminder (units per chain): a poly(methyl propenoate) sample has average Mr 1.29 × 10⁵. Mr(C₄H₆O₂) = 4(12.0) + 6(1.0) + 2(16.0) = 86.0, so units = 1.29 × 10⁵ ÷ 86.0 = 1500.

Property	Reason
Unreactive	Saturated: no C=C left; strong, non-polar C–C and C–H bonds
Solid at room temperature	Very long chains, very many electrons, large contact → large total van der Waals attraction
Extra forces in PVC	Polar C–Cl → permanent dipole–dipole forces too

PVC (poly(chloroethene)):

Form	Property	Typical uses
Unplasticised	rigid	water and sewage pipes, doors, bank cards
Plasticised	soft, flexible	cable insulation, inflatable products, flooring

Plasticiser (commonly phthalate esters) → molecules between chains → chains further apart → weaker intermolecular forces → chains slide → flexible.

Development over time: PVC was first made in 1872 (Eugen Baumann) but was hard to use; plasticising it (Waldo Semon, B.F. Goodrich, 1926) made it flexible and widely useful.

3.3.4.4 Epoxyethane

Point	Key fact
Structure	Three-membered ring: two CH₂ groups and one O
Production	2CH₂=CH₂ + O₂ → 2C₂H₄O; silver catalyst; about 220–280 °C, 1–3 MPa
Atom economy	100% (one product)
Hazards	Competing combustion is far more exothermic; product extremely flammable, explosive with air, toxic, carcinogenic
High reactivity	Ring angles about 60° (not 109.5°) → strained, weak C–O bonds; polar Cδ+–Oδ− attracts nucleophiles

Equations:

C₂H₄O + H₂O        → HOCH₂CH₂OH          ethane-1,2-diol
C₂H₄O + CH₃CH₂OH   → CH₃CH₂OCH₂CH₂OH     2-ethoxyethanol
ROH + nC₂H₄O       → R(OCH₂CH₂)ₙOH        surfactant when R is a long chain

Method: mechanism with water or an alcohol

1. Curly arrow starts at an oxygen lone pair of H₂O or ROH and ends at a ring carbon.
2. Arrow from that ring C–O bond to the ring O (ring opens).
3. Intermediate RO⁺(H)CH₂CH₂O⁻; a proton moves from O⁺–H to O⁻.

Industrial notes: a little dilute acid speeds up the water reaction; a large excess of water limits further reaction of the diol with more epoxyethane.

Product	Importance
Ethane-1,2-diol	Antifreeze and engine coolant; making polyester (PET); toxic if swallowed, but breaks down in the environment within weeks
Glycol ethers	Solvents in paints and cleaners
Long-chain ethoxylates	Non-ionic surfactants in detergents; harmful to aquatic life, so ready breakdown in water matters

Must-know distinctions

• Electrophile vs nucleophile: alkenes are attacked by electrophiles; epoxyethane is attacked by nucleophiles (water, alcohols).
• HBr vs Br₂ on an unsymmetrical alkene: HBr gives major and minor products; Br₂ gives one.
• Carbocation stability vs product stability: the major product is explained by the intermediate, not by the product.
• Within vs between chains: unreactivity is about covalent bonds in the chain; solidity and flexibility are about forces between chains.
• Monomer vs repeating unit: same formula, but only the monomer has C=C.

Quick self-test

1. Define an electrophile.
2. Explain why Br₂, a non-polar molecule, can act as an electrophile.
3. Name the product of hept-1-ene with bromine.
4. Which of oct-1-ene and oct-4-ene gives two products with HBr? Explain.
5. Through which atom does the hydrogensulfate ion bond to the carbocation?
6. Give the monomer for the chain –CH₂–CH(CN)–CH₂–CH(CN)–.
7. Name the polymer made from CH₂=CF₂.
8. Explain why adding a plasticiser makes PVC flexible.
9. State the catalyst used to make epoxyethane from ethene.
10. Write the equation for epoxyethane reacting with ethanol and name the product.
11. Give two reasons why epoxyethane is very reactive.
12. Give one use of ethane-1,2-diol made from epoxyethane.

Answers

1. An electron pair acceptor.
2. The C=C electron density repels the Br–Br bonding electrons, inducing a dipole; the nearer Br becomes δ+.
3. 1,2-dibromoheptane, CH₃(CH₂)₄CHBrCH₂Br.
4. Oct-1-ene: it is unsymmetrical, so two different carbocations (secondary and primary) can form. Oct-4-ene is symmetrical and gives only 4-bromooctane.
5. The negatively charged oxygen atom.
6. CH₂=CHCN, propenenitrile.
7. Poly(1,1-difluoroethene).
8. Small plasticiser molecules lodge among the chains and space them out; the forces between chains become weaker and the chains can move over one another.
9. Silver.
10. C₂H₄O + CH₃CH₂OH → CH₃CH₂OCH₂CH₂OH; 2-ethoxyethanol.
11. Strained ring (bond angles about 60°) with weak C–O bonds; polar C–O bonds make the carbons δ+.
12. Antifreeze (or engine coolant, or making PET).

Where marks are usually lost

• Starting the opening arrow on one of the alkene carbons instead of the middle of the double bond.
• No induced dipole shown on Br₂.
• Third arrow drawn from the minus sign rather than from a lone pair on the anion.
• Positive charge put on the carbon that does not match the product named.
• Major products “explained” by the stability of the product rather than the carbocation.
• Repeating unit drawn with C=C, without extension bonds, or with a substituent on the wrong carbon.
• Polymer unreactivity blamed on strong intermolecular forces.
• Epoxyethane hazards given only as “dangerous”, with no specific hazard (flammable, explosive, toxic, carcinogenic, exothermic side reaction).
• Epoxyethane ring opening drawn with the arrow from the ring to water rather than from water’s lone pair to the ring carbon.
• The final proton transfer left out of the epoxyethane mechanism.

Official syllabus

OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards: section 3.3.4 Alkenes (International AS), points 3.3.4.1 to 3.3.4.4.