Reactions
55.3.6 Electrophilic addition
Most addition reactions to alkenes follow the mechanism of electrophilic addition. An
example is the Prins reaction^8 , where the electrophile is a carbonyl group.
Halogenation
Addition of elementary bromine or chlorine to alkenes yield vicinal dibromo- and
dichloroalkanes, respectively.
The decoloration of a solution of bromine in water is an analytical test for the presence of
alkenes: CH 2 =CH 2 + Br 2 → BrCH 2 -CH 2 Br
The reaction works because the high electron density at the double bond causes a temporary
shift of electrons in the Br-Br bond causing a temporary induced dipole. This makes the
Br closest to the double bond slightly positive and therefore an electrophile.
Hydrohalogenation
Addition of hydrohalic acids like HCl or HBr to alkenes yield the corresponding haloalkanes.
an example of this type of reaction: CH 3 CH=CH 2 + HBr → CH 3 -CHBr-CH 3If the two carbon atoms at the double bond are linked to a different number of hydrogen
atoms, the halogen is found preferentially at the carbon with less hydrogen substituents
(Markovnikov’s rule).
Addition of a carbene or carbenoid yields the corresponding cyclopropane
55.3.7 Oxidation
Alkenes are oxidized with a large number of oxidizing agents. In the presence of oxygen,
alkenes burn with a bright flame to carbon dioxide and water. Catalytic oxidation with
oxygen or the reaction with percarboxylic acids yields epoxides.
Reaction with ozone in ozonolysis leads to the breaking of the double bond, yielding two
aldehydes or ketones: R 1 -CH=CH-R 2 + O 3 → R 1 -CHO + R 2 -CHO + H 2 O
This reaction can be used to determine the position of a double bond in an unknown alkene.
55.3.8 Polymerization
Polymerization of alkenes is an economically important reaction which yields polymers of
high industrial value, such as the plastics polyethylene and polypropylene. Polymerization
can either proceed via a free-radical or an ionic mechanism.