Alkenes

Chemical Properties of Alkenes

Chemical Properties of Alkenes

Due to the C=C bond, alkenes have a number of different chemical properties to alkanes.

Addition reactions

The ?-bonds in alkenes can be broken in order for ?-bonds with other species to be formed without atoms being forced to break off. This means that alkenes are able to undergo addition reactions, unlike alkanes. Molecules that contain ?-bonds and can, therefore, take part in addition are known as unsaturated. On the other hand, molecules which consist of ?-bonds but cannot take part in addition are known as saturated. Addition reactions tend to be quicker than substitution reactions as only weak ?-bonds have to be broken as opposed to ?-bonds which are stronger.

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Heterolytic reactions

Alkenes contain an area of high electron density: the ?-bond. Therefore, it it able to attract electrophiles (a species able to accept a pair of electrons from a species which has a high electron density) and take part in heterolytic fission. In heterolytic fission a covalent bond is broken and both electrons go to the same atom. Alkanes, on the other hand, are only able to react with free radicals and take part in homolytic fission.

However, alkenes also undergo homolytic fissions as the bond is non-polar. Therefore, alkenes can react in two ways:

  • by free radical addition
  • by electrophilic addition

This make alkenes much more reactive than alkanes.

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Electrophilic addition reaction of alkenes

The C=C ?-bond usually undergoes electrophilic addition when electrophiles are present. Electrophiles include:

  • hydrogen halides (H-Cl, H-I and H-Br)
  • halogens (Br-Br, Cl-Cl and I-I)

It is also possible for alkenes to take part in an electrophilic addition reaction with H2SO4.

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Hydrogen halides

The positive dipole on the H in the H-X bond is attacked by the pair of electrons located on the C=C bond. This then undergoes heterolytic fission. The carbocation is then attacked by the halide ion to create a haloalkane. The reaction is quite fast and can take place at room temperature.

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Halogens

A halogen is non-polar but when an alkene is present the electron moves over to one side of the molecule putting into a state of temporary dipole. This ?+ve state causes the alkene to attack, forming a dibromoalkane. For example: CH2=CH2 + Br2 ? CH2BrCH2Br.

During the reaction the Br2 changes colour from orange to colourless. In fact, a good test for an alkene is to add bromine solution. When the mixture is shaken it will decolourise if alkenes are present.

The reaction is quite fast and, again, can take place at room temperature.

H2SO4

With cold concentrated sulphuric acid alkenes will also undergo an electrophilic addition reaction.

If the mixture is warmed up and water added an OH group replaces the H2SO4 group to produce alcohol. This is called a hydrolysis reaction as water is used to break the covalent bonds. In the laboratory it is an effective way of converting alkenes into alcohols.