Organic chemistry studies carbon compounds. This section covers homologous series (families of compounds), crude oil fractionation, and reactions of key organic compounds.
Crude oil: mixture of hydrocarbons separated by fractional distillation (based on boiling points). Fractions: refinery gas, petrol, naphtha, kerosene, diesel, fuel oil, bitumen. Alkanes CₙH₂ₙ₊₂: saturated hydrocarbons (single C-C bonds only). Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀. As chain length increases: boiling point increases, viscosity increases, less flammable. Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O (complete).
Alkenes CₙH₂ₙ: unsaturated, contain C=C double bond. Ethene C₂H₄, propene C₃H₆. Test: bromine water decolourised by alkenes (addition reaction). Addition reactions: hydrogenation (+H₂), hydration (+H₂O → alcohol). Addition polymerisation: many small alkene molecules (monomers) join to form long chains (polymers). Poly(ethene), poly(propene), PVC. Cracking: breaking long alkanes into shorter alkanes + alkenes (thermal or catalytic).
Alcohols: -OH functional group. Methanol CH₃OH, ethanol C₂H₅OH. Ethanol made by: fermentation (glucose → ethanol + CO₂, yeast, ~37°C, anaerobic) or hydration of ethene. Used as fuel, solvent. Carboxylic acids: -COOH group. Ethanoic acid CH₃COOH (vinegar). Properties: weak acids, react with alcohols to form esters (+ acid catalyst). Esters: used in flavourings and perfumes. Condensation polymers: form nylon, polyester (with small molecule like H₂O released).
Cracking is the process of breaking down long-chain hydrocarbons into shorter, more useful molecules. It produces shorter alkanes (for fuels like petrol) and alkenes (for making polymers). This is important because crude oil contains too many large molecules (heavy fractions) and not enough small molecules (light fractions like petrol). Thermal cracking uses high temperatures (600-700°C) and pressure. Catalytic cracking uses a zeolite catalyst at lower temperatures (~500°C). This process matches supply with demand for petroleum products.
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