Organic Chemistry: Alkanes and Alkenes IGCSE – Structure, Reactions and Tests
Organic chemistry: alkanes and alkenes is one of the most searched IGCSE Chemistry topics because it appears in almost every paper. You need to know structure, general formulae, reactions (combustion, cracking for alkanes; addition for alkenes), and tests (e.g. bromine for alkenes).
Alkanes – Key Points
- Saturated – Single C–C bonds only; general formula (C_nH_{2n+2}).
- Combustion – Burn to give CO₂ and H₂O (complete); may give CO or C if limited oxygen.
- Cracking – Long-chain alkane → shorter alkane + alkene; conditions (catalyst/temperature) often required in answers.
Alkenes – Key Points
- Unsaturated – At least one C=C double bond; general formula (C_nH_{2n}).
- Bromine test – Orange bromine decolourises with alkene (addition); no change with alkane.
- Addition reactions – e.g. with hydrogen (to form alkane), steam (to form alcohol); know conditions where needed.
Learn structural and displayed formulae for the first few members so you can draw and name them correctly.
Exam-Style Questions and Common Mistakes
You may be asked to name alkanes/alkenes, draw structures, write equations (combustion, addition), or explain the bromine test. For “explain why alkenes decolourise bromine”, link the C=C double bond to addition: bromine adds across the double bond, so the orange colour is lost. Common mistakes: Confusing alkane and alkene formulae (alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ); wrong combustion products (complete: CO₂ and H₂O; incomplete: CO or C if limited oxygen); forgetting conditions for reactions (e.g. catalyst for addition of hydrogen). Practise with past paper organic questions and mark schemes.
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