OxfordAQA International A Level Chemistry 3.3.4 Alkenes Exam Questions

About OxfordAQA International A Level Chemistry 3.3.4 Alkenes

Alkenes is a Unit 2 organic topic about compounds with a C=C double bond. You need to explain why alkenes undergo electrophilic addition and draw mechanisms for reactions with hydrogen bromide, sulfuric acid and bromine. You also need to use carbocation stability to explain major and minor products, and use bromine water as a test for unsaturation. The OxfordAQA specification includes addition polymers, PVC and plasticisers, and the production of epoxyethane by partial oxidation of ethene, with its reactions with water and alcohols to make antifreeze and surfactants and the hazards involved. It links to halogenoalkanes, alcohols and polymers at A2.

What you need to know

• Structure and reactivity The C=C double bond is a region of high electron density, so it attracts electrophiles, which are electron pair acceptors.
• Electrophilic addition With HBr, the pi bond attacks the H of H-Br, forming a carbocation and Br-, which then bonds to the carbocation. Bromine and concentrated sulfuric acid also add across the double bond.
• Major and minor products For unsymmetrical alkenes, the major product forms via the more stable carbocation. Tertiary carbocations are more stable than secondary, which are more stable than primary, because alkyl groups release electrons.
• Test for alkenes Bromine water is decolourised from orange to colourless by an alkene.
• Addition polymers Alkenes join to form addition polymers, which are unreactive because they are saturated. PVC is made from chloroethene, and plasticisers are added to make it flexible by allowing chains to slide over each other.
• Epoxyethane Epoxyethane is made by partial oxidation of ethene with oxygen and a silver catalyst. It reacts with water to make ethane-1,2-diol (antifreeze) and with alcohols to make surfactants; it is toxic and flammable, so it must be handled carefully.

Common mistakes in 3.3.4 Alkenes

• Drawing the first curly arrow from a carbon atom. It starts from the C=C double bond and goes to the electrophile.
• Saying the minor product is formed via the more stable carbocation. The major product comes from the more stable carbocation.
• Saying bromine water goes clear. Say it changes from orange to colourless.
• Saying addition polymers are unreactive because they are long. They are unreactive because the chains are saturated with strong C-C and C-H bonds.

Try three 3.3.4 Alkenes exam-style questions