CIE A Level Chemistry Hydrocarbons Exam Questions

About CIE A Level Chemistry Hydrocarbons

Hydrocarbons in Cambridge International 9701 covers alkanes and alkenes at AS and arenes at A Level. For alkanes you study combustion, the environmental effects of pollutants, cracking and the free radical substitution mechanism with chlorine or bromine in UV light. For alkenes you need electrophilic addition mechanisms with bromine, hydrogen halides and steam, Markovnikov addition explained by carbocation stability, oxidation by acidified potassium manganate(VII) and addition polymerisation. The A Level part covers the structure and stability of benzene, electrophilic substitution such as nitration and Friedel-Crafts reactions, and the oxidation of side chains. Examiners repeatedly ask for full mechanisms with correct curly arrows and partial charges.

What you need to know

• Alkanes and combustion Alkanes are saturated and fairly unreactive because C-C and C-H bonds are strong and non-polar. Incomplete combustion produces CO and carbon, and catalytic converters reduce CO, NO and unburnt hydrocarbons.
• Free radical substitution Initiation: Cl2 → 2Cl• in UV light. Propagation: CH4 + Cl• → CH3• + HCl, then CH3• + Cl2 → CH3Cl + Cl•. Termination: two radicals combine. Further substitution gives a mixture of products.
• Cracking Long-chain alkanes are broken into shorter alkanes and alkenes using heat and a catalyst such as Al2O3. This meets demand for petrol and provides alkenes for polymers.
• Electrophilic addition Alkenes react with Br2, HBr and H2O (steam with H3PO4 catalyst). The pi electrons attack the electrophile, forming a carbocation intermediate. Bromine water decolourising is the test for C=C.
• Markovnikov addition With unsymmetrical alkenes, the major product forms from the more stable carbocation. Tertiary carbocations are more stable than secondary, then primary, because alkyl groups are electron donating.
• Oxidation of alkenes Cold dilute acidified KMnO4 gives a diol. Hot concentrated acidified KMnO4 cleaves the C=C, giving ketones, carboxylic acids or CO2 depending on the groups attached.
• Arenes (A Level) Benzene has a delocalised ring of pi electrons, making it more stable than expected, so it undergoes substitution not addition. Nitration uses concentrated HNO3 and H2SO4 to form NO2+; halogenation uses AlCl3 or FeBr3; Friedel-Crafts alkylation and acylation use AlCl3.

Common mistakes in Hydrocarbons

• Writing an initiation step that produces H• from CH4; UV light breaks the Cl-Cl bond.
• Drawing the curly arrow in alkene addition from a carbon atom rather than from the C=C double bond.
• Forgetting to show the lone pair on the bromide ion attacking the carbocation.
• Saying benzene decolourises bromine water; it does not react because addition would destroy the stable delocalised ring.
• Mixing up cold dilute and hot concentrated KMnO4 products.

Try three Hydrocarbons exam-style questions