Organic chemistry: alkanes and alkenes

Alkanes and alkenes are the first families of organic chemistry — chains of carbon and hydrogen. Alkanes are saturated and fairly unreactive; alkenes have a carbon–carbon double bond that makes them reactive, and one quick test tells them apart.

Homologous series

Each family is a homologous series — compounds with the same general formula and similar chemistry, differing by CH₂ each step. Alkanes (methane, ethane, propane…) are saturated (single bonds only); alkenes (ethene, propene…) are unsaturated, with one carbon–carbon double bond.

Telling them apart

The standard test is bromine water: an alkene decolourises it (orange to colourless) as it adds across the double bond, while an alkane leaves it orange. This single observation is a frequent exam ask and the reason the double bond defines alkene chemistry.

Reactions

Alkanes mainly burn — complete combustion gives carbon dioxide and water, incomplete combustion gives carbon monoxide or soot. Alkenes undergo addition reactions at the double bond (with bromine, hydrogen, or steam to make alcohols), and many alkene molecules can join into long chains by addition polymerisation to make plastics. (The detailed polymerisation mechanism is not required.)

Where they come from

Both come from crude oil, separated by fractional distillation. Long, less useful alkane fractions are broken into smaller, more valuable molecules — including alkenes — by cracking, which uses heat and a catalyst. This links the topic to industry and to the air-quality issues of burning fuels.