Organic Chemistry
Homologous series
A homologous series is a family of organic compounds that:
- Share the same general formula.
- Differ by one –CH₂– unit between successive members.
- Show a gradual trend in physical properties (boiling point, viscosity).
- Have similar chemical properties (react in similar ways).
Alkanes
General formula: CₙH₂ₙ₊₂. Single C–C bonds only (saturated).
| Name | Formula | Structural |
|---|---|---|
| Methane | CH₄ | CH₄ |
| Ethane | C₂H₆ | CH₃CH₃ |
| Propane | C₃H₈ | CH₃CH₂CH₃ |
| Butane | C₄H₁₀ | CH₃(CH₂)₂CH₃ |
Reactions:
- Combustion: complete combustion in excess O₂ gives CO₂ + H₂O; incomplete (limited O₂) gives CO (toxic) and/or soot C. CH₄ + 2O₂ → CO₂ + 2H₂O (complete)
- Substitution (halogenation): with Cl₂ or Br₂ in UV light. H atoms replaced one at a time. CH₄ + Cl₂ → CH₃Cl + HCl (chloromethane)
Alkenes
General formula: CₙH₂ₙ. Contains a C=C double bond (unsaturated).
| Name | Formula |
|---|---|
| Ethene | C₂H₄ |
| Propene | C₃H₆ |
Test for unsaturation: decolourise bromine water (orange → colourless). Alkanes do NOT decolourise bromine water (no reaction without UV).
Reactions:
- Addition reactions: the double bond opens and two reactants add across it.
- Hydrogenation: C₂H₄ + H₂ → C₂H₆ (Ni catalyst, 150 °C)
- Hydration: C₂H₄ + H₂O → C₂H₅OH (H₃PO₄ catalyst, 300 °C, 60 atm)
- Halogenation: C₂H₄ + Br₂ → CH₂BrCH₂Br (dibromoethane)
Alcohols
General formula: CₙH₂ₙ₊₁OH. Functional group: –OH.
| Name | Formula |
|---|---|
| Methanol | CH₃OH |
| Ethanol | C₂H₅OH |
| Propanol | C₃H₇OH |
Reactions:
- Combustion: burns in O₂ to give CO₂ + H₂O.
- Oxidation: primary alcohol → aldehyde → carboxylic acid (using acidified K₂Cr₂O₇; orange → green).
- Esterification: alcohol + carboxylic acid ⇌ ester + water (H₂SO₄ catalyst, reflux, reversible).
Fermentation (production of ethanol): C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (yeast, ~30–40 °C, anaerobic)
Carboxylic acids
General formula: CₙH₂ₙ₊₁COOH. Functional group: –COOH.
- Ethanoic acid (CH₃COOH) — vinegar.
- Weak acids (only partially ionised in solution).
- React with alcohols to form esters (see above).
- React with carbonates to produce CO₂.
Polymers
Addition polymers: formed from alkene monomers; no other product.
- Poly(ethene) from ethene: n(C₂H₄) → –(CH₂CH₂)ₙ–
- Poly(propene) from propene; poly(chloroethene)/PVC from chloroethene; poly(styrene) from phenylethene.
Condensation polymers: formed with loss of a small molecule (usually H₂O).
- Polyesters: diol + dicarboxylic acid → polyester + H₂O. Example: PET (from ethane-1,2-diol + benzene-1,4-dicarboxylic acid).
- Polyamides (nylons): diamine + diacid chloride → polyamide + HCl. Example: Nylon-6,6.
Cracking of hydrocarbons
Long-chain alkanes from crude oil are cracked (thermal or catalytic cracking) into shorter alkanes + alkenes. Alkenes are monomers for addition polymers.
C₁₀H₂₂ → C₈H₁₈ + C₂H₄
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