Producing Alkanes
A concise Cambridge International AS Level Chemistry revision guide to the two ways of producing alkanes named in 14.1.1: the addition of hydrogen to an alkene with a platinum or nickel catalyst and heat, and the cracking of a longer-chain alkane by heating with aluminium oxide.
GCSE Recap: Formulae and Balancing
Before you start, check the general formulae and the atom counting this page depends on.
Two Routes to an Alkane
Cambridge 14.1.1 asks you to recall the reagents and conditions for two reactions that produce alkanes. Both are met again elsewhere in the syllabus, but here the alkane is the product you are asked about.
| Route | Starting material | Reagents and conditions | Type of reaction | Example |
|---|---|---|---|---|
| Hydrogenation | An alkene | H2(g) with a Pt or Ni catalyst, heat | Addition | CH3CH=CH2 + H2 → CH3CH2CH3 |
| Cracking | A longer-chain alkane | Heat with Al2O3 | Thermal decomposition (C-C bonds broken) | C10H22 → C8H18 + C2H4 |
Producing alkanes: adding hydrogen across the C=C bond of an alkene, or cracking a longer alkane over hot aluminium oxide.
Key idea: Hydrogenation makes an alkane from a smaller, unsaturated molecule; cracking makes an alkane (and an alkene) from a larger, saturated one.
Quick Check: Picking the Right Route
Decide which starting material and route would give one pure alkane.
Hydrogenation of Alkenes
Hydrogen adds across the C=C double bond of an alkene in the presence of a platinum or nickel catalyst with heat. The π bond breaks, two new C-H σ bonds form, and the product is the saturated alkane with the same carbon skeleton. The catalyst provides a surface on which the H2 molecule is split into atoms that add to the alkene.
Because the carbon skeleton is unchanged, the alkane formed is easy to predict: but-1-ene and but-2-ene both give butane; 2-methylpropene gives 2-methylpropane. The reaction is used industrially to make saturated fats from vegetable oils, where the same catalyst and conditions apply.
Exam sentence: Alkenes are converted to alkanes by addition of hydrogen: H2(g) with a platinum or nickel catalyst and heat.
Quick Check: Predict the Product
Work each answer out on paper, then flip the card to compare.
Cracking a Longer Alkane
Heating a long-chain alkane with aluminium oxide, Al2O3, breaks a C-C bond and produces a shorter alkane together with an alkene. The Cambridge wording is simply “heat with Al2O3“; the oxide acts as a catalyst. Any C-C bond can break, so a mixture forms, but in each equation the carbon and hydrogen atoms must balance and one product is an alkene.
- C12H26 → C7H16 + C5H10 (heptane and pentene)
- C12H26 → C8H18 + 2C2H4 (octane and two ethene molecules)
Cracking is also how refineries obtain more useful alkanes and alkenes of lower relative molecular mass from the heavy fractions of crude oil (14.1.4); the full industrial context is on the Cracking page.
Exam focus: Quote the conditions exactly as the syllabus does: heat with Al2O3 for cracking; H2 with Pt or Ni and heat for hydrogenation.
Quick Check: Finish the Cracking Equation
Balance the equation to find the formula of the missing alkene.
Quick Check: Pick the Accurate Statement
In each round, choose the one statement that is accurate.
Common Exam Mistakes
- Giving the hydrogenation catalyst as “a catalyst” without naming platinum or nickel.
- Writing a cracking equation with two alkanes as products. One product must be an alkene for the hydrogen atoms to balance.
- Describing hydrogenation as a substitution. It is an addition reaction: the alkene gains two hydrogen atoms and nothing is lost.
- Confusing the cracking catalyst (Al2O3) with the hydrogenation catalyst (Pt or Ni).
Exam sentence: Alkanes are produced by hydrogenating an alkene with H2 over Pt or Ni with heat, or by cracking a longer alkane by heating with Al2O3.
Master Introduction to Organic Chemistry and Alkanes for Cambridge International AS & A Level Chemistry
Continue from these free revision notes into the full Topics 13 and 14.1 Introduction to Organic Chemistry and Alkanes course. The guided video lessons are ready now; the Cambridge International MCQ bank, teacher-marked short-answer questions and KASP spec-point report are being written, and the course opens for enrolment as soon as they are complete.
Guided video teaching
Learn the chemistry and exam technique through structured video lessons with worked examples and walkthroughs.
Instant MCQ feedback
Auto-marked MCQ quizzes provide immediate diagnostic feedback for every answer choice.
Teacher-marked SAQs
Submit written exam responses and receive chemistry specialist feedback with improvement guidance.
Progress tracking
Identify strengths and weaknesses across the full Topics 13 and 14.1 specification with targeted reporting.
See how the course works
Click play to start the course preview animation.
Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Free Radical Substitution FAQs
These questions summarise the core exam points on alkane substitution and free radicals.
Why are alkanes usually unreactive?
Alkanes are usually unreactive because they contain strong C-C and C-H bonds. These bonds require a large amount of energy to break.
What condition is needed for alkanes to react with chlorine?
Ultraviolet light is needed. UV light provides enough energy to break the Cl-Cl bond by homolytic fission, forming chlorine radicals.
What is a free radical?
A free radical is a species with an unpaired electron. The unpaired electron is usually represented using a dot, such as Cl•.
Why is the reaction called substitution?
It is called substitution because a hydrogen atom in the alkane is replaced by a halogen atom.
Why can a mixture of products form?
After the first substitution, the haloalkane product can undergo further substitution. This can replace more hydrogen atoms and produce a mixture of chlorinated products.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
Keep this note — free
Save your progress across every Cambridge topic. A free account remembers which topics you have covered, saves your question scores, and syncs across your phone and laptop.
- Track every topic you have finished
- Keep your practice-question scores
- No payment, no card, free forever
Already registered? Log in