AQA A-Level Chemistry: Alkenes (7405)

This guide teaches 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. It covers all three sub-sections: 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. The specification places organic sections 3.3.1 to 3.3.6 on AS Paper 2 and all of section 3.3 on A-level Paper 2, and A-level Paper 3 can test any content.

For quick recall, use the alkenes revision notes. To test yourself, try the alkenes practice questions. The course hub is AQA A-Level Chemistry, the printable checklist lists every specification point, and the free 10-minute diagnostics show where your gaps are. Van der Waals forces and dipole–dipole forces are taught in bonding, and moles and masses in the mole revision notes.

What this topic covers

Spec section	What you must be able to do
3.3.4.1	State that alkenes are unsaturated hydrocarbons
3.3.4.1	Explain that the C=C double bond is a centre of high electron density
3.3.4.2	Outline electrophilic addition mechanisms with HBr, H₂SO₄ and Br₂
3.3.4.2	Use bromine to test for unsaturation
3.3.4.2	Explain major and minor products from unsymmetrical alkenes using primary, secondary and tertiary carbocation stability
3.3.4.3	Describe addition polymers made from alkenes and substituted alkenes
3.3.4.3	Draw the repeating unit from a monomer, or from a section of chain; draw the monomer from a section of chain
3.3.4.3	Name addition polymers using IUPAC rules
3.3.4.3	Explain why addition polymers are unreactive, and the intermolecular forces between polyalkene molecules
3.3.4.3	Describe typical uses of PVC and how a plasticiser modifies it
3.3.4.3	Appreciate that understanding of polymer production and properties has developed over time

Section 3.3.4 itself lists no required practical. The bromine test for an alkene is part of Required practical 6 (tests for alcohol, aldehyde, alkene and carboxylic acid), which the specification lists under 3.3.6.1.

Structure, bonding and reactivity (3.3.4.1)

Alkenes are unsaturated hydrocarbons: they contain only carbon and hydrogen, and they have at least one carbon–carbon double bond. Ethene, CH₂=CH₂, is the simplest. A non-cyclic alkene with one double bond has the general formula CₙH₂ₙ.

A double bond has two shared pairs of electrons between the same two carbon atoms. That makes the C=C bond a centre of high electron density. Species that are short of electrons are attracted to it. These are electrophiles: electron pair acceptors, such as H⁺ or the δ+ end of a polar molecule.

This explains the typical reaction of alkenes. An electrophile attacks the double bond, one of the two bonds breaks, and two new single bonds form. The result is addition: two species add across the double bond and the product is saturated.

Electrophilic addition (3.3.4.2)

Every mechanism in this section follows the same two steps.

1. The electrophile accepts a pair of electrons from the C=C bond. A bond in the electrophile breaks heterolytically, and a carbocation (an ion with a positive carbon) forms with a negative ion.
2. The negative ion uses a lone pair to bond to the positive carbon.

Curly arrows show the movement of a pair of electrons. Each arrow starts at a bond or a lone pair and ends at the atom (or bond position) the electrons move to.

With hydrogen bromide

H–Br is polar: Hδ+–Brδ−. The δ+ hydrogen is the electrophile. The reaction happens at room temperature.

Step 1   H₂C=CH₂  +  Hδ+–Brδ−   →   H₃C–CH₂⁺  +  Br⁻
         arrow 1: from the C=C bond to the H of H–Br
         arrow 2: from the H–Br bond to the Br atom

Step 2   H₃C–CH₂⁺  +  :Br⁻   →   H₃C–CH₂Br   (bromoethane)
         arrow 3: from a lone pair on Br⁻ to the positive carbon

With bromine, and the test for unsaturation

Br₂ is not polar. As a bromine molecule approaches the high electron density of the double bond, the electrons in Br–Br are repelled away from the nearer atom. This induces a dipole: the nearer Br atom becomes δ+ and acts as the electrophile.

Step 1   H₂C=CH₂  +  Brδ+–Brδ−   →   BrH₂C–CH₂⁺  +  Br⁻
         arrow 1: from the C=C bond to the δ+ Br
         arrow 2: from the Br–Br bond to the δ− Br

Step 2   BrH₂C–CH₂⁺  +  :Br⁻   →   BrH₂C–CH₂Br   (1,2-dibromoethane)

Test for unsaturation. Shake the compound with bromine water at room temperature. An alkene decolourises it: orange to colourless, because the bromine is used up in addition. A saturated compound such as an alkane leaves the solution orange.

With sulfuric acid

Cold, concentrated sulfuric acid adds across the double bond. One H of H₂SO₄ is the electrophile.

Step 1   H₂C=CH₂  +  H–OSO₂OH   →   H₃C–CH₂⁺  +  ⁻OSO₂OH
         arrow 1: from the C=C bond to an H of H₂SO₄
         arrow 2: from that O–H bond to the O atom

Step 2   H₃C–CH₂⁺  +  ⁻OSO₂OH   →   CH₃CH₂OSO₂OH   (ethyl hydrogensulfate)
         arrow 3: from a lone pair on the negative O to the positive carbon

The negative O of the hydrogensulfate ion, not the S or an OH, bonds to the carbon. Making alcohols by hydrating alkenes is taught in the alcohols study guide.

Major and minor products (unsymmetrical alkenes)

An unsymmetrical alkene has different groups on the two carbons of the double bond, for example propene, CH₃CH=CH₂. With HBr or H₂SO₄, the electrophile can add to either carbon, so two carbocations are possible and two products form.

Carbocations are classified by how many alkyl groups are bonded to the positive carbon:

Type	Alkyl groups on C⁺	Example	Stability
primary	1	CH₃CH₂CH₂⁺	least stable
secondary	2	CH₃CH⁺CH₃	more stable
tertiary	3	(CH₃)₃C⁺	most stable

Alkyl groups release electron density towards the positive carbon. This spreads out (reduces) the positive charge and stabilises the ion. More alkyl groups give more stability. The more stable carbocation forms more readily, so most of the product forms through it. That product is the major product; the other is the minor product.

Worked example 1. Propene reacts with hydrogen bromide. Name the major and minor products and explain which is major.

• If H adds to the end carbon (C1), the positive charge is on C2: CH₃CH⁺CH₃, a secondary carbocation. Br⁻ then bonds to C2, giving 2-bromopropane.
• If H adds to C2, the positive charge is on C1: CH₃CH₂CH₂⁺, a primary carbocation. Br⁻ bonds to C1, giving 1-bromopropane.
• The secondary carbocation has two electron-releasing alkyl groups, so it is more stable than the primary carbocation and forms more readily.
• Major product: 2-bromopropane. Minor product: 1-bromopropane.

Bromine does not give major and minor products. Both atoms that add are Br, so either route leads to the same compound (1,2-dibromopropane from propene). A symmetrical alkene such as but-2-ene also gives one product with HBr, because both carbocations are identical.

Addition polymers (3.3.4.3)

Alkenes and substituted alkenes (alkenes with another atom or group in place of an H, such as chloroethene, CH₂=CHCl) join together in addition polymerisation. The double bonds open and the monomers bond into a long saturated chain. Nothing else is produced.

Repeating units, monomers and names

The repeating unit is the two-carbon unit that came from one monomer. To draw it, open the double bond, keep every substituent on the same carbon, and draw extension bonds from both carbons. Put brackets round it and an n outside.

monomer               repeating unit

  H   Cl                  H   Cl
  |   |                   |   |
  C = C              —(— C — C —)—n
  |   |                   |   |
  H   H                   H   H

To go back from a section of chain, find the two-carbon pattern that repeats, then put the double bond back between those two carbons to get the monomer.

IUPAC naming: write poly, then the monomer’s name in brackets. Ethene gives poly(ethene); propene gives poly(propene); chloroethene gives poly(chloroethene); phenylethene gives poly(phenylethene).

Worked example 2. A section of an addition polymer is –CH₂–C(CH₃)₂–CH₂–C(CH₃)₂–CH₂–C(CH₃)₂–. Give the repeating unit, the monomer and the polymer’s name.

• The repeating two-carbon unit is –CH₂–C(CH₃)₂–.
• Put the double bond back: monomer CH₂=C(CH₃)₂, which is 2-methylpropene.
• Polymer name: poly(2-methylpropene).

Worked example 3. A sample of poly(chloroethene) has an average Mr of 1.25 × 10⁵. How many monomer units are in an average chain? (Ar: H = 1.0, C = 12.0, Cl = 35.5)

• Mr of C₂H₃Cl = 2(12.0) + 3(1.0) + 35.5 = 62.5. The repeating unit has the same formula as the monomer.
• Number of units = 1.25 × 10⁵ / 62.5 = 2000.

Why addition polymers are unreactive

The polymer chain is saturated. It contains only strong, non-polar (or almost non-polar) C–C and C–H bonds, and there is no double bond left for electrophiles to attack. So polyalkenes are unreactive.

Intermolecular forces in polyalkenes

The chains in poly(ethene) and poly(propene) are non-polar, so the forces between them are van der Waals forces. Each chain is very long and has a very large number of electrons, and neighbouring chains are in contact along their length. So, although each interaction is weak, the total attraction between chains is large. That is why these polymers are solids. In poly(chloroethene) the C–Cl bond is polar, so there are also permanent dipole–dipole forces between the chains.

PVC and plasticisers

Poly(chloroethene) is commonly known as PVC. Unplasticised PVC is rigid; typical uses include window frames, drainpipes and guttering. A plasticiser is a small molecule mixed into the polymer. Its molecules get between the chains and push them further apart. This weakens the intermolecular forces between chains, so the chains slide past each other more easily and the PVC becomes flexible. Plasticised PVC is used, for example, for electrical cable insulation and flooring.

How understanding developed over time

The specification asks you to appreciate that knowledge of polymer production and properties has developed over time. One example: early poly(ethene) was made at very high pressure, which gives branched chains. Later, catalysts allowed poly(ethene) to be made at lower pressure with much less branching. Less-branched chains pack more closely, so the forces between them are stronger and the material is denser and stiffer. Learning how to modify PVC with plasticisers is another example: the same polymer can be given very different properties.

Practical opportunity

The specification suggests making poly(phenylethene) from phenylethene (AT k, PS 1.2). This is not a required practical.

Common errors

• Starting the first curly arrow from a carbon atom instead of from the double bond.
• Leaving out the δ+/δ− on H–Br, or the induced dipole on Br₂.
• Drawing the second arrow from the negative ion’s charge sign rather than from a lone pair.
• Calling the more stable carbocation “more reactive”, or saying alkyl groups “attract” electrons.
• Claiming that bromine gives major and minor products with propene.
• Leaving the double bond in a repeating unit, or forgetting the extension bonds and n.
• Putting a substituent on the wrong carbon, so the repeating unit becomes three carbons long.
• Saying polyalkenes are unreactive because “the bonds are weak”.

Where next

Use the alkenes revision notes, then the alkenes practice questions and the exam preparation guide.

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).