Contents
1. Introduction to Organic Chemistry
Organic chemistry is the study of carbon-containing compounds. Carbon forms four covalent bonds and can bond to other carbon atoms to form chains, branches and rings.
A homologous series is a family of compounds with: the same general formula; the same functional group; similar chemical properties; physical properties that change gradually with chain length.
| Series | General formula | Functional group | Example |
|---|---|---|---|
| Alkanes | CₙH₂ₙ₊₂ | None (C−C single bonds only) | CH₄, C₂H₆, C₃H₈ |
| Alkenes | CₙH₂ₙ | C=C double bond | C₂H₄, C₃H₆ |
| Alcohols | CₙH₂ₙ₊₁OH | −OH (hydroxyl) | CH₃OH, C₂H₅OH |
| Carboxylic acids | CₙH₂ₙ₊₁COOH | −COOH (carboxyl) | HCOOH, CH₃COOH |
2. Alkanes
Alkanes are saturated hydrocarbons — they contain only C−C single bonds and C−H bonds. They are relatively unreactive because C−C and C−H bonds are strong.
| Name | Formula | Carbons |
|---|---|---|
| Methane | CH₄ | 1 |
| Ethane | C₂H₆ | 2 |
| Propane | C₃H₈ | 3 |
| Butane | C₄H₁₀ | 4 |
Reactions of alkanes
- Combustion (complete): alkane + O₂ → CO₂ + H₂O. e.g. CH₄ + 2O₂ → CO₂ + 2H₂O
- Combustion (incomplete): insufficient oxygen → CO (toxic) and/or C (soot) produced instead of CO₂.
- Substitution with halogens: in UV light, H atoms are replaced one at a time by halogen atoms. e.g. CH₄ + Cl₂ → CH₃Cl + HCl (UV light required; no reaction in dark).
Cracking
Cracking breaks long-chain alkanes into shorter, more useful alkanes and alkenes. Conditions: high temperature (400–700°C) and a catalyst (aluminium oxide / silica).
C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (octane + ethene)
The alkene (C₂H₄) can be used to make polymers. The shorter alkane (C₈H₁₈) is petrol.
3. Alkenes
Alkenes are unsaturated hydrocarbons — they contain at least one C=C double bond. The double bond makes alkenes much more reactive than alkanes.
Test for alkene
Add bromine water — alkene decolourises it (orange/brown → colourless) due to addition reaction across the double bond. Alkanes do not decolourise bromine water.
Addition reactions
| Reagent | Conditions | Product |
|---|---|---|
| H₂ (hydrogen) | Ni catalyst, 150°C (hydrogenation) | Alkane (C=C → C−C) |
| Br₂ (bromine) | Room temperature | Dibromoalkane (decolourises bromine water) |
| H₂O (steam) | H₃PO₄ catalyst, 300°C, high pressure | Alcohol (hydration) |
| HCl / HBr | Room temperature | Haloalkane |
Alkenes undergo addition (the double bond opens; two atoms/groups add across it; one product formed). Alkanes undergo substitution (one H is replaced; two products formed). Never say alkenes undergo substitution — they don't.
4. Alcohols
Alcohols contain the −OH (hydroxyl) functional group. The most important at O-Level are methanol (CH₃OH) and ethanol (C₂H₅OH).
Production of ethanol
| Method | Conditions | Advantage | Disadvantage |
|---|---|---|---|
| Fermentation (C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂) | Yeast enzyme, 25–35°C, anaerobic | Renewable; low energy cost | Slow; dilute product; impure |
| Hydration of ethene (C₂H₄ + H₂O → C₂H₅OH) | H₃PO₄ catalyst, 300°C, 60 atm | Fast; pure product; continuous | Uses non-renewable crude oil; high energy |
Reactions of alcohols
- Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O (complete combustion)
- Oxidation to carboxylic acid: ethanol → ethanoic acid (e.g. wine → vinegar)
- Dehydration to alkene: C₂H₅OH → C₂H₄ + H₂O (Al₂O₃ catalyst, heat)
- Esterification with carboxylic acids → ester + water
5. Carboxylic Acids
Carboxylic acids contain the −COOH (carboxyl) functional group. They are weak acids — partially ionised in solution.
- Methanoic acid: HCOOH
- Ethanoic acid: CH₃COOH (acetic acid / vinegar)
Reactions of carboxylic acids
- With metals: acid + metal → salt + hydrogen (e.g. 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂)
- With bases/alkalis: acid + alkali → salt + water
- With carbonates: acid + carbonate → salt + water + CO₂
- Esterification: carboxylic acid + alcohol → ester + water (concentrated H₂SO₄ catalyst, heat, reversible)
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Ethanoic acid + ethanol → ethyl ethanoate + water
Ethyl ethanoate is a sweet-smelling ester used in food flavourings and solvents.
6. Polymers
Many small monomer molecules (alkenes) join together by opening their C=C double bonds to form a long polymer chain. No other product is formed.
n(CH₂=CH₂) → −(CH₂−CH₂)ₙ−
Many ethene monomers → poly(ethene) chain. The double bond opens and becomes a single bond linking the repeat units.
| Monomer | Polymer | Use |
|---|---|---|
| Ethene (CH₂=CH₂) | Poly(ethene) / polythene | Plastic bags, bottles |
| Propene (CH₂=CHCH₃) | Poly(propene) | Ropes, crates, carpets |
| Chloroethene (CH₂=CHCl) | PVC | Pipes, window frames |
| Styrene (C₈H₈) | Polystyrene | Packaging, insulation |
The repeat unit is the smallest section that repeats along the polymer chain. For poly(ethene), it is −CH₂−CH₂−. Draw brackets around it with an n subscript and bonds emerging from each bracket end.
- Alkanes (CnH2n+2): saturated, only C-C single bonds. Complete combustion: CO2 + H2O. Substitution with Cl2 in UV light.
- Alkenes (CnH2n): unsaturated, contain C=C. Undergo addition reactions (Br2 water, H2, HCl). Test: decolourise bromine water.
- Cracking: breaking long-chain alkanes into shorter alkanes + alkenes. Needs high temperature and catalyst.
- Esterification: alcohol + carboxylic acid -> ester + water. Reversible. Needs acid catalyst (H2SO4) and heat.
- Addition polymer: alkene monomer, double bond opens, long chain forms. e.g. chloroethene -> PVC.
- Condensation polymer: monomers with two functional groups react, releasing small molecules (H2O or HCl). e.g. nylon, polyester.
7. Common Exam Traps
This is the standard test to distinguish alkanes from alkenes. In UV light, alkanes also react with bromine — but the question will specify no UV light for the test to be valid.
Yeast ferments sugar in the absence of oxygen. If oxygen is present, complete combustion of ethanol occurs instead of fermentation. The reaction vessel must be sealed but CO₂ must be able to escape.
The ester and water can react in the reverse direction (hydrolysis). To get a good yield, remove the ester as it forms or use excess of one reactant. Students often forget the reverse arrow or that the reaction does not go to completion.
Ethyl ethanoate = ethanol + ethanoic acid. The alcohol part gives the first name (ethyl); the acid part gives the second name (ethanoate). Always name the alcohol-derived part first.
Key Terms — Flashcard Review
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🎯 Practice Quiz — Test Yourself
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.