Alkene

One large molecule can form several smaller molecules, so the products are a mixture. They include alkanes, whose carbon atoms have single bonds between them, and alkenes, which contain a carbon–carbon double bond, C=C.

An alkene is therefore unsaturated. “Unsaturated” does not mean that the molecule lacks hydrogen altogether; it means that it contains at least one carbon–carbon multiple bond, here a double bond.

A name can provide a check: -ane indicates an alkane, -ene an alkene, -ol an alcohol, and -oic acid a carboxylic acid. The beginning of the name tells you about the number of carbon atoms, while the ending tells you the family. For example, ethanol and ethanoic acid both have two carbon atoms, but their endings—and their functional groups—place them in different homologous series.

In catalytic cracking, the hydrocarbon is vaporised and passed over a hot catalyst. Its molecules split into shorter hydrocarbons: the products include smaller alkanes and alkenes. Alkenes contain a carbon–carbon double bond, which makes them more reactive than alkanes.