Edexcel International A Level Chemistry

Topic 5: Alkenes

The first unsaturated family. The C=C double bond fixes the shape of the molecule, gives geometric isomers, and makes alkenes react by addition, with a mechanism you can draw. Alkenes also join to make polymers.

The one idea

An alkene is a hydrocarbon with a C=C double bond. The double bond has two parts. The σ bond is like the single bonds of an alkane. The π bond is a weaker region of electron density above and below the plane of the bond. That π bond does three jobs. It stops rotation, which is why some alkenes have geometric isomers. It is electron-rich, so it is attacked by electrophiles. And it can break open, so an alkene undergoes addition with hydrogen, halogens, hydrogen halides and steam, and joins to other alkenes to make polymers.

Everything on these pages comes from that one bond. If you meet an alkene you have never seen, ask what the π bond will do.

Where to find things

How to use these notes

• Read the pages in order the first time. The geometric isomers (pages 2 to 4) come first because they come straight from the shape of the C=C. The mechanism pages (8 to 12) build on each other, and page 12 needs page 11.
• Each page opens with its one idea in a sentence. If you can explain it to someone else, you know the page.
• Coloured boxes mean the same thing on every page: definitions you may be asked to quote, key ideas, common mistakes, exam technique, and beyond the spec (interesting, but you will not be examined on it).
• Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
• Use the topic on a page the night before an exam.

This topic has no core practical. The tests on page 7 (bromine water and acidified manganate(VII)) are described as practical boxes because the observations are common exam questions, but they are not listed practicals.

The skills that earn most marks here are drawing a mechanism with every curly arrow, dipole, charge and lone pair in the right place; naming E and Z isomers by priority; explaining the major product through carbocation stability; drawing a repeat unit with brackets and extension bonds; and giving observations (colour changes) and conditions for each reaction.

You need the naming and isomerism of Topic 4 and the covalent bonding of Topic 3 for this topic. Where an earlier topic is needed, the page says so.

Pages

1. Alkenes and the double bond

   Alkenes are unsaturated hydrocarbons with general formula CₙH₂ₙ. Their C=C double bond is one σ bond plus one π bond, and the exposed π electrons make alkenes reactive.

   13 min 5.1

2. Restricted rotation around C=C

   Groups can spin around a C–C single bond but not around a C=C double bond, because twisting would break the π bond. This locks the groups in place and gives geometric isomers.

   11 min 5.2

3. Which alkenes have geometric isomers

   Geometric isomers need a C=C and two different groups on each carbon of it. Test each carbon of the double bond separately, then draw the isomers with 120° bond angles.

   13 min 5.2

4. E–Z naming

   The E–Z system names geometric isomers by the priority of the groups on each carbon of the C=C. It works for every alkene with isomers, including those where cis and trans break down.

   14 min 5.3

5. Adding hydrogen and halogens

   Alkenes react by addition. The π bond breaks and two atoms add across the C=C. Hydrogen with a nickel catalyst gives an alkane, and a halogen gives a di-substituted halogenoalkane.

   14 min 5.45.4(i)5.4(ii)

6. Adding hydrogen halides and steam

   A hydrogen halide or steam adds H to one carbon of the C=C and X or OH to the other. A symmetrical alkene gives one product; an unsymmetrical alkene gives a major and a minor product.

   14 min 5.45.4(iii)5.4(iv)

7. Oxidation to a diol and the bromine test

   Acidified potassium manganate(VII) oxidises a C=C to a diol and turns from purple to colourless. Bromine water is decolourised by a C=C, which makes it the test for an alkene.

   13 min 5.45.4(v)5.5

8. Electrophiles and curly arrows

   The π bond is a region of high electron density, so it attracts electrophiles. Before drawing any mechanism, you need the rules for curly arrows and for how a bromine molecule can act as an electrophile.

   12 min 5.6

9. The mechanism for ethene and bromine

   Bromine adds to ethene in two steps. The π electrons attack an induced Br⁺ end, forming a carbocation, and then a bromide ion attacks it. The products formed in bromine water are the evidence for two steps.

   14 min 5.65.6(i)

10. The mechanism for ethene and hydrogen bromide

    Hydrogen bromide adds to ethene by the same two-step pattern as bromine. The difference is that H–Br is permanently polar, so no dipole has to be induced.

    11 min 5.65.6(i)

11. Carbocation stability

    Carbocations are primary, secondary or tertiary according to the number of alkyl groups on the positive carbon. Alkyl groups release electrons, so tertiary is the most stable and primary the least.

    12 min 5.6

12. The mechanism for propene and hydrogen bromide

    Propene can form a secondary or a primary carbocation with HBr. The secondary carbocation is more stable, so 2-bromopropane is the major product and 1-bromopropane the minor product.

    13 min 5.65.6(ii)

13. Addition polymerisation

    Many alkene monomers join to make one long saturated chain. The C=C of each monomer becomes C–C, nothing else is formed, and the repeat unit is the part of the chain that repeats.

    12 min 5.7

14. From monomer to repeat unit, and back

    Draw the repeat unit by turning the C=C into C–C and keeping the side groups in place. Work backwards by finding the two carbons of one unit and putting the C=C back.

    13 min 5.7

15. Polymer disposal

    Addition polymers are unreactive and do not rot. Chemists develop biodegradable polymers, and remove the toxic gases from incineration by passing them through a base in a scrubber.

    11 min 5.85.8(i)5.8(ii)

Also in this topic: Topic 5 on a page

Spec checklist

Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.