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 C_nH_2n. 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.

 

Ethene

See main entry.

 

Ethene (C₂H₄), also known as ethylene, is the first member of the alkene family. It is a colorless, flammable gas, with a slightly sweet smell. Ethene burns in air with a luminous flame and forms an explosive mixture with pure oxygen.

 

Propene

Propene (C₃H₆), also known as propylene, is the second-simplest alkene. About two thirds of the propene derived from the petroleum industry is used to produce polypropene. Propene and benzene are converted to acetone and phenol via the cumene process.

 

Butene

Butene, also called butylene, is any of three alkene gases having the general formula C₄H₈: butene-1 (CH₃CH₂CH=CH₂), butene-2 (CH₂CH=CHCH₃), and isobutene ([CH₃]₂C=CH₂) or 2-methyl-propene. Butene is made from petroleum and used as a starting material for other organic compounds.

 

Butadiene

Butadiene, also known as 1,3 butadiene (CH₂=CHCH=CH₂), is a gaseous, flammable hydrocarbon made by dehydrogenation of butane and butene or by cracking naptha. Butadiene, an alkene, is mostly copolymerized with styrene to make synthetic rubber. Relative density 0.62, melting point –108.9°C ( –164.0°F), boiling point -4.4°C (24.1°F). See also Diels-Alder reaction.

 

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.