What are the properties of alkenes?

Structural features

Alkenes are organic compounds that contain a double carbon-carbon (alkenyl) bond. They are said to be ‘unsaturated’ in that they can add more hydrogen atoms to the hydrocarbon skeleton.

The double carbon-carbon bond is a region of high electron density, with one pair of electrons in a ‘pi’ orbital, which can be attacked by reagents containing a positive or partial positive charge. This makes alkenes reactive and useful as building blocks for organic synthesis.

Naming alkenes

The position of the double bond is numbered using the carbon with the smallest number in the chain. If there is branching the chain is chosen that includes the double bond.

Example: Name the following alkene:

The longest chain with the double bond has three carbon atoms. The chain is numbered to keep the double bond with the lowest number. The bromine, therefore is on carbon number 3. Hence 3-bromopropene

Electrophilic addition reactions

Alkenes can form addition products with other molecules by ‘opening’ the double bond and using the electrons to form bonds at each carbon atom. This is known as addition.

The process is stimulated by electrophilic attack by the reagent, and for this reason is called electrophilic addition.

Alkenes react with halogens making disubstituted halogenoalkanes

ethene + bromine 1,2-dibromoethane

C2H4 + Br2 C2H4Br2

Alkenes react with interhalogens making disubstituted halogenoalkanes

ethene + iodine monochloride 1-chloro-2-iodoethane

C2H4 + ICl CH2ClCH2I

Alkenes react with hydrogen halides making halogenoalkanes:

ethene + hydrogen bromide bromoethane

C2H4 + HBr C2H5Br

Alkenes react with steam in the presence of a catalyst making alcohols:

ethene + steam ethanol

C2H4 + H2O C2H5OH

Alkenes react with hydrogen in the presence of a Ni catalyst at 150ºC making alkanes:

ethene + hydrogen ethane

C2H4 + H2 C2H6

Although it may seem pointless making an unreactive alkane from its useful alkane, this reaction is important in synthesis, as it allows control of the degree of unsaturation of long chain compounds that have several double bonds, such as the vegetable oils.

Changing double bonds to single bonds, via hydrogenation, increases the melting point of a vegetable oil, an essential step in the production of margarine.