The simplest alkenes.
Alkenes are unsaturated hydrocarbons that contain one or more double carbon-carbon bonds. Alkenes with just one double C-C bond (monoalkenes) form an homologous series with the general formula CnH2n. The first few members of this series are ethene, propene, butene, and pentene, their names being derived from those of the corresponding alkanes by replacing the suffix –ane by –ene. Higher members of the series show isomerism depending on the position of the double bond. For example, butene (C4H8) has two isomers – but-1-ene (C2H5CH:CH2) and but-2-ene (CH3CH:CHCH3).
Alkenes are are prepared by thermal cracking of alkanes (from petroleum or natural gas), dehydration of alcohols, or base-catalyzed elimination of hydrogen halides from alkyl halides. Alkenes physically resemble alkanes but the presence of at least one double bond in alkenes means that they react in ways that alkanes cannot do. Many reagents add across the double bond: hydrogen (with nickel or platinum catalyst), halogens, hydrogen halides, and sulfuric acid. Alkenes are oxidized by permanganate or hypochlorite to glycols, and by ozone to ozonides, which readily decompose to aldehydes and ketones.
The double bond also allows alkenes to be distinguished from alkanes using a simple chemical test. This test involves the use of bromine water – a dilute solution of bromine which is normally orange-brown in color. Bromine water becomes colorless when shaken with an alkene, through an addition reaction, but is not decolorized by alkanes.
Alkene polymers and plastics
Under pressure and in the presence of a suitable catalyst, small alkenes will join together, by the opening up of their double bonds, to form long chains called polymers. The joining up of many individual alkene units by polymerization results in a variety of much-used and familiar plastics. These include polythene, polypropene, epoxy, polystyrene, and polychloroethene (PV).
Polythene, made from ethene, is very cheap and strong, and is easily molded. It is used to make, for example, plastic bags, bottles, and buckets. Polypropene, made from propene, has strong fibers and a high elasticity. It is used, for example, in the manufacture of crates, ropes, and carpets. Epoxy resins are polymers that can be used as adhesives. Polystyrene, made from styrene, is cheap, easily molded, and can be expanded into insulating forma. Polychloroethene (PVC) is made from chloroethene and used for making wire insulation and records.