Crude oil provides the starting materials for the manufacture of plastics. The simplest of these starting materials is ethene, an alkene, which is made from some of the fractions of crude oil by a process called cracking . There is a double covalent bond between two of the carbon atoms in each molecule of an alkene. These double bonds give the alkenes their characteristic chemistry. The naming system of alkenes follows the same pattern as that for alkanes, but there is no methene as a minimum of two carbon atoms is needed for there to be a double bond.
Alkenes burn in air or oxygen, as do alkanes.
The characteristic reaction of alkenes is addition. The double bond opens up and substances such as hydrogen, chlorine, bromine are added on. Alkenes often polymerise to form polymers, many of which are useful plastics e.g. polythene from ethene, polypropylene from propene.