Organic Chemistry (Section 4) covers the chemistry of carbon compounds — crude oil, hydrocarbons (alkanes and alkenes), alcohols, carboxylic acids, and polymers.
Crude oil: mixture of hydrocarbons separated by fractional distillation (based on boiling point). Shorter chain → lower BP, more volatile, more flammable, less viscous. Fractions: gases, petrol, kerosene, diesel, fuel oil, bitumen. Alkanes CₙH₂ₙ₊₂: saturated (single bonds only). Complete combustion: CₙH₂ₙ₊₂ + O₂ → CO₂ + H₂O. Incomplete: produces CO (toxic) and/or C (soot). Cracking: breaks long-chain into shorter useful ones. Thermal cracking (high temp, high pressure) → alkenes for polymers. Catalytic cracking (zeolite catalyst, lower temp) → branched alkanes for fuel.
Alkenes CₙH₂ₙ: unsaturated (C=C double bond). Test: decolourises bromine water. Addition reactions: + Br₂ → dibromoalkane, + H₂ → alkane, + H₂O (with acid catalyst) → alcohol, + HBr → bromoalkane. Alcohols (−OH): ethanol by fermentation (glucose + yeast → ethanol + CO₂) or hydration of ethene (+ steam, H₃PO₄ catalyst). Uses: solvent, fuel. Oxidation → carboxylic acid. Carboxylic acids (−COOH): weak acids. React with carbonates → salt + water + CO₂. Esterification: alcohol + carboxylic acid → ester + water (acid catalyst). Addition polymers: many alkene monomers join at the double bond (e.g., ethene → polyethene). Condensation polymers: nylon, polyester (monomers linked losing small molecule like water).
Addition polymerisation: monomers have a C=C double bond (alkenes). The double bond opens and monomers link together. No other molecule is produced. The polymer has the same atoms as the monomer — no by-products. Example: ethene → poly(ethene). Condensation polymerisation: monomers have two functional groups (e.g., dicarboxylic acid + diol → polyester, or diamine + dicarboxylic acid → nylon). Each time two monomers join, a small molecule (usually H₂O) is lost. The polymer has fewer atoms than the total monomers. Key exam distinction: addition → no loss of atoms, monomer needs C=C. Condensation → loss of small molecule (H₂O), monomers need two functional groups.
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