Organic chemistry — hydrocarbons
What is organic chemistry?
Organic chemistry is the study of carbon-containing compounds. Carbon can form four covalent bonds and bond to itself in long chains, giving enormous variety.
Hydrocarbon: compound containing only hydrogen and carbon.
Alkanes — saturated hydrocarbons
Alkanes have only single bonds between carbon atoms. General formula: CₙH₂ₙ₊₂.
| Name | Formula |
|---|---|
| Methane | CH₄ |
| Ethane | C₂H₆ |
| Propane | C₃H₈ |
| Butane | C₄H₁₀ |
Alkanes are saturated — all bonds are single bonds; no more hydrogen can be added.
Property trends as chain length increases: boiling point increases; viscosity increases; flammability decreases.
Crude oil and fractional distillation
Crude oil is a mixture of hydrocarbons with different chain lengths, separated by fractional distillation:
- Crude oil heated → vaporised.
- Vapour enters fractionating column (temperature decreases up the column).
- Hydrocarbons condense at different levels depending on boiling point.
- Short chains condense near top; long chains near bottom.
| Fraction | Chain length | Use |
|---|---|---|
| Refinery gases | C1−C4 | Fuel (LPG) |
| Petrol | C5−C10 | Car fuel |
| Kerosene | C11−C15 | Jet fuel |
| Diesel | C16−C25 | Lorry fuel |
| Bitumen | C70+ | Road surfacing |
Cracking — making shorter, more useful molecules
Demand for petrol is higher than the amount in crude oil. Long-chain alkanes are surplus. Cracking breaks long chains into shorter molecules.
Catalytic cracking: hot hydrocarbon vapour over a catalyst (aluminium oxide/zeolite) at ~500°C.
Products of cracking include:
- Shorter alkanes (more useful fuels).
- Alkenes (for making polymers).
Example: C₁₆H₃₄ → C₈H₁₈ + C₄H₈ + C₄H₈
Alkenes — unsaturated hydrocarbons
Alkenes contain at least one C=C double bond. General formula: CₙH₂ₙ.
| Name | Formula |
|---|---|
| Ethene | C₂H₄ |
| Propene | C₃H₆ |
Alkenes are unsaturated — they undergo addition reactions (double bond opens, additional atoms add across it).
Test for alkenes: add bromine water (orange/brown). Alkenes decolourise it (turn colourless) because bromine adds across the C=C. Alkanes do NOT decolourise bromine water.
Addition polymerisation
Alkene monomers join together to form polymers. The double bond in each monomer opens, allowing them to link.
Example: many ethene (CH₂=CH₂) → poly(ethene): −(CH₂−CH₂)ₙ−
Uses: poly(ethene) — plastic bags; poly(propene) — crates, ropes; PVC — pipes, flooring.
AI-generated · claude-opus-4-7 · v3-ccea-combined-science