Alkenes are unsaturated hydrocarbons with at least one $C=C$ double bond. Alkenes:


$CH_{2}=CH_{2}$ is one of the most important alkenes. It is obtained from the catalytic cracking of crude oil fractions. The $C=C$ double bond is the functional group in the alkene that is responsible for its reactions.
$$Cl_{2} \rightarrow Cl\cdot + Cl\cdot$$

Nature of the double bond

The $C=C$ bond is made up of two parts: a sigma ($\sigma$) bond and a pi ($\pi$) bond.

In alkenes, each carbon involved in the double bond:


Shape of an alkene molecule

Addition reactions of Alkenes

Addition of hydrogen

A mixture of hydrogen gas $H_{2}$ and a gaseous alkene is passed over a catalyst of nickel at a temperature of 150°. This is sometimes known as a hydrogenation reaction.
$$CH_{2}{=}CH_{2} + H_{2} \rightarrow CH_{3}CH_{3}$$

Addition of halogens

Alkenes react rapidly with halogens such as chlorine, bromine and iodine at room temperature. This reaction sometimes known as a halogenation reaction, forms a di-substituted halogenoalkane.
$$CH_{2}{=}CH_{2} + Cl_{2} \rightarrow CH_{2}ClCH_{2}Cl$$

Addition of hydrogen halides

A hydrogen halide can be added across the double bond of an alkene to produce a halogenoalkane. Halogen halides include $HCl$, $HBr$ and $HI$. At room temperature, hydrogen halides are gases and can be bubbled through liquid alkenes.
$$CH_{2}{=}CH_{2} + HBr \rightarrow CH_{3}CH_{2}Br$$

Addition of unsymmetrical alkenes

Unsymmetrical alkenes such as propene can form a mixture of organic products. This is because the atoms can be added at either side of the double bond.

Addition of steam

The addition of steam to alkenes is one method of producing alcohols. This reaction is widely used in industry.

Electrophilic addition reactions

Addition of hydrogen bromide

Hydrogen bromide, $HBr$ is a polar molecule. This is because bromine is more electronegative than hydrogen. This causes the dipole $H^{\delta+}{-}Br^{\delta-}$


The product of the reaction is a halogenalkane, in this case bromoethane. In the mechanism above, $HBr$ is the electrophile.

Addition of bromine

Bromine reacts with alkenes at room temperature, producing a di-substituted halogenoalkane.

Test for unsaturation

When $Br_{2(aq)}$ is added to an alkene the colour changes from orange to colourless. This colour change indicates that it has reacted with a alkene.

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© Andrew Deniszczyc, 2024