Alkanes (A-Level) — A-Level Chemistry Revision
Revise Alkanes (A-Level) for A-Level Chemistry. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel and OCR.
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Go to Alkenes (A-Level)What is Alkanes (A-Level)?
Alkanes are saturated hydrocarbons that are the main components of crude oil. At A-Level, the focus is on their fractional distillation, cracking (to produce more useful shorter-chain alkanes and alkenes), and reformation. A key reaction is their free-radical substitution with halogens in the presence of UV light, which proceeds via a three-step mechanism: initiation, propagation, and termination.
Board notes: All boards cover the mechanism of free-radical substitution in detail. AQA often asks about the environmental impact of using fossil fuels and the role of catalytic converters. Edexcel may focus on the industrial processes of cracking and reforming. OCR questions frequently require students to predict the products of free-radical substitution and explain why a mixture of products is formed.
Step-by-step explanationWorked example
Show the propagation steps for the reaction of methane with chlorine. Step 1: A chlorine radical attacks a methane molecule, abstracting a hydrogen atom to form a methyl radical and a molecule of hydrogen chloride. Cl- + CH4 -> -CH3 + HCl. Step 2: The methyl radical is also very reactive and attacks a chlorine molecule, forming chloromethane and a new chlorine radical. -CH3 + Cl2 -> CH3Cl + Cl-. This new chlorine radical can then repeat step 1, continuing the chain reaction.
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Common mistakes
- 1Confusing the conditions for cracking with those for free-radical substitution. Cracking requires high temperatures and a catalyst, whereas free-radical substitution requires UV light.
- 2Incorrectly drawing the mechanism for free-radical substitution, for example by using double-headed arrows (which show movement of an electron pair) instead of single-headed arrows (for movement of a single electron).
- 3Forgetting that free-radical substitution can lead to a mixture of products, as substitution can occur at different positions on the carbon chain and multiple substitutions can happen.
Alkanes (A-Level) exam questions
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Alkanes (A-Level)
Core concept
Alkanes are saturated hydrocarbons that are the main components of crude oil. At A-Level, the focus is on their fractional distillation, cracking (to produce more useful shorter-chain alkanes and alke…
Frequently asked questions
Why are alkanes relatively unreactive?
Alkanes are unreactive because they only contain strong, non-polar C-C and C-H sigma bonds. There are no regions of high electron density to attract electrophiles, and the bonds require a large amount of energy to break.
What is the difference between thermal and catalytic cracking?
Thermal cracking uses high temperatures (around 700-1200K) and high pressures (up to 7000 kPa) and produces a high proportion of alkenes. Catalytic cracking uses a lower temperature and pressure but requires a zeolite catalyst, and tends to produce aromatic compounds and motor fuels.