IGCSE Chemistry Organic Chemistry Notes
Organic chemistry is the chemistry of carbon compounds. Most start life in crude oil and are used as fuels, plastics and chemicals.
Key concepts
- Homologous series
- Family of compounds with the same general formula, similar properties and gradually changing physical properties.
- Alkanes
- Saturated hydrocarbons, CₙH₂ₙ₊₂. Fuels; burn to give CO₂ and H₂O.
- Alkenes
- Unsaturated hydrocarbons with C=C, CₙH₂ₙ. Decolourise bromine water; used to make polymers.
- Alcohols
- Contain −OH group, e.g. ethanol; used as fuels, solvents and in drinks.
- Carboxylic acids
- Contain −COOH group, e.g. ethanoic acid; weak acids.
- Fractional distillation separates crude oil into fractions by boiling point.
- Cracking breaks long-chain alkanes into shorter, more useful alkanes and alkenes.
- Alkenes + bromine water: orange → colourless (test for C=C).
- Addition polymerisation joins alkene monomers into long saturated polymer chains.
Worked example
Write the equation for the complete combustion of ethane (C₂H₆).
- 1Products of complete combustion: CO₂ + H₂O.
- 2Skeleton: C₂H₆ + O₂ → CO₂ + H₂O
- 3Balance C: 2CO₂; balance H: 3H₂O.
- 4Balance O: 7 O on the right, so 3.5 O₂ (or 7/2) on the left. Multiply through by 2 if needed.
Answer: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
Common mistakes
✗ Writing the alkene general formula as CₙH₂ₙ₊₂.
✓ Alkanes are CₙH₂ₙ₊₂; alkenes are CₙH₂ₙ.
✗ Claiming complete combustion produces carbon monoxide.
✓ Complete combustion gives CO₂ and H₂O only; CO comes from incomplete combustion.
✗ Forgetting that addition polymers are saturated.
✓ The C=C opens up during polymerisation, so the polymer chain has only single bonds.
Quick check
Try these, then reveal the answers.
What is the test for an alkene?Reveal answer
Add bromine water; it goes from orange to colourless.
What is the functional group of an alcohol?Reveal answer
−OH (hydroxyl).
Name the monomer used to make poly(ethene).Reveal answer
Ethene (C₂H₄).
Exam tips
- Draw displayed formulae showing every bond and atom.
- For polymer questions, draw the repeat unit inside brackets with 'n' subscripts.
- State observation AND conclusion for the bromine water test.
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Frequently asked questions
Why is cracking useful?+
It converts less useful long-chain alkanes into shorter alkanes (better fuels) and alkenes (for polymers).
What is a hydrocarbon?+
A compound made of only hydrogen and carbon atoms.
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