GCSE Organic Chemistry — Alkanes, Alkenes, Polymers and Cracking

← Back to Blog

Organic chemistry at GCSE covers the chemistry of carbon compounds — a huge area of modern chemistry that includes fuels, plastics, medicines and foods. The topic has a reputation for being confusing because students try to memorise individual facts rather than understanding the underlying patterns. This guide focuses on those patterns — once you understand the logic of each homologous series, every question becomes manageable.

What Makes a Compound Organic?

Organic compounds are compounds that contain carbon. Most also contain hydrogen, and many contain oxygen, nitrogen or halogens too. Carbon is unusual among elements because it can form four covalent bonds and can bond to other carbon atoms to form long chains, rings and branched structures — giving rise to millions of different compounds.

Alkanes — The Simplest Hydrocarbons

Alkanes are hydrocarbons — compounds containing only carbon and hydrogen — in which all bonds are single bonds. They are said to be saturated because they contain the maximum possible number of hydrogen atoms for the number of carbons present.

The general formula for alkanes is CₙH₂ₙ₊₂. The first four members:

Alkanes form a homologous series — a family of compounds with the same general formula, the same functional group, and similar chemical properties, where each successive member differs by CH₂. Properties change gradually along the series: as chain length increases, melting point, boiling point and viscosity all increase (because there are more electrons and therefore stronger intermolecular forces between molecules).

Combustion of Alkanes

Alkanes burn in oxygen. Complete combustion (plenty of oxygen) produces carbon dioxide and water only:

Incomplete combustion (limited oxygen) produces carbon monoxide (CO) and/or carbon (soot) instead of CO₂. Carbon monoxide is toxic — it binds to haemoglobin in red blood cells, preventing oxygen transport. This is why gas appliances must be properly ventilated.

Crude Oil and Fractional Distillation

Crude oil is a mixture of hydrocarbons of different chain lengths. It is separated by fractional distillation — a process that exploits the fact that shorter-chain hydrocarbons have lower boiling points than longer-chain ones.

Crude oil is heated until it vaporises. The vapour enters a fractionating column, which is cooler at the top and hotter at the bottom. Each fraction condenses at a different height and is collected. Shorter chains (e.g. petrol, kerosene) condense near the top; longer chains (e.g. diesel, fuel oil, bitumen) condense near the bottom.

Cracking — Breaking Long Chains Into Shorter Ones

There is more demand for shorter-chain hydrocarbons (petrol, kerosene) than for long-chain ones. Cracking converts long-chain alkanes into shorter, more useful molecules. It also always produces at least one alkene — which is valuable for making polymers.

There are two types of cracking:

• Thermal cracking: High temperature (400–900°C) and high pressure. Produces mostly alkenes.
• Catalytic cracking: Lower temperature (450°C) with a zeolite catalyst. More economical. Produces branched alkanes (better for petrol) and aromatic compounds as well as alkenes.

Alkenes — Unsaturated Hydrocarbons

Alkenes contain at least one carbon-carbon double bond (C=C). They are unsaturated — they contain fewer hydrogen atoms than the maximum possible. The general formula is CₙH₂ₙ.

The double bond makes alkenes much more reactive than alkanes. They undergo addition reactions — the double bond opens up and atoms add across it, producing a single-bonded product.

Addition Reactions of Alkenes

• Hydrogenation: Alkene + H₂ → alkane. Nickel catalyst, 150°C. Used to make margarine from vegetable oils (hardening).
• Hydration: Alkene + H₂O → alcohol. Steam, phosphoric acid catalyst, 300°C. Used industrially to make ethanol.
• Halogenation: Alkene + Br₂ → dibromoalkane. This is the bromine water test — the orange/brown bromine is decolourised as it adds across the double bond.

Polymers — Addition Polymerisation

Many small alkene molecules (monomers) can join together to form very long chain molecules (polymers). This is addition polymerisation — the double bonds in the monomers open up and the molecules link together, with no atoms lost in the process.

The repeat unit in the polymer is written in square brackets with n outside — this represents one unit repeated n times along the chain. To draw the repeat unit from a monomer: remove the double bond and add a bond on each side. To identify the monomer from a polymer: add a double bond between the two carbons that were previously connected by single bonds.

Condensation Polymerisation (Higher Tier)

Unlike addition polymerisation, condensation polymerisation uses two different types of monomer and produces a small molecule (usually water or HCl) as a byproduct each time two monomers join.

Nylon is a condensation polymer made from a diamine and a dicarboxylic acid. Each time an amine group (−NH₂) reacts with a carboxylic acid group (−COOH), an amide bond forms and a water molecule is released. Polyester (e.g. PET, used in plastic bottles and clothing) is made from a diol and a dicarboxylic acid — each join produces an ester bond and releases water.

The AQA organic chemistry specification is at the AQA GCSE Chemistry specification page.