Organic chemistry — section overview

Organic chemistry is the study of carbon-containing compounds. Section C7 covers hydrocarbons (alkanes and alkenes), alcohols, carboxylic acids, esters, and polymer chemistry.

Hydrocarbons

Alkanes: CₙH₂ₙ₊₂ (saturated — all single bonds)

• Methane CH₄, Ethane C₂H₆, Propane C₃H₈, Butane C₄H₁₀

Alkenes: CₙH₂ₙ (unsaturated — at least one C=C double bond)

• Ethene C₂H₄, Propene C₃H₆

Test for alkenes: add bromine water; decolourises (turns from orange-brown to colourless) as bromine adds across the C=C bond.

Combustion of hydrocarbons

• Complete combustion (excess O₂): hydrocarbon + O₂ → CO₂ + H₂O
• Incomplete combustion (limited O₂): produces CO (toxic) and carbon (soot)

Crude oil and fractional distillation

Crude oil is a mixture of hydrocarbons separated by fractional distillation. Smaller molecules (shorter chains): lower boiling point, more flammable, less viscous, more volatile.

Cracking: breaking down long-chain alkanes into shorter, more useful alkanes + alkenes (using high temperature and catalyst/steam).

Alcohols

General formula: CₙH₂ₙ₊₁OH. Example: ethanol (C₂H₅OH).

• Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
• Fermentation: glucose + yeast → ethanol + CO₂ (at ~25–35°C, anaerobic)
• Reaction with carboxylic acids → ester + water (condensation/esterification)

Carboxylic acids

General formula: CₙH₂ₙ₊₁COOH. Example: ethanoic acid (CH₃COOH — vinegar). Weak acids; react with metals, carbonates and alcohols.

Esters

Formed when alcohol + carboxylic acid react (with acid catalyst). Used as perfumes and food flavourings. Example: ethanol + ethanoic acid → ethyl ethanoate + water.

Addition and condensation polymerisation

Addition polymerisation: many alkene monomers join together — C=C bonds open up. n(CH₂=CH₂) → (−CH₂−CH₂−)ₙ (poly(ethene))

Condensation polymerisation (HT): monomers with two functional groups join, releasing small molecules (H₂O or HCl).

Common exam mistakes in C7

1. Alkanes are saturated — all single bonds — alkenes are unsaturated (C=C)
2. Bromine water test — colour lost with alkenes — alkanes do NOT decolourise bromine water
3. Fermentation — anaerobic, ~25°C — too hot = enzyme denatured; too cold = slow