11.6 - Reactions of Alkenes

Alkenes and addition reactions

• All alkenes contain the functional group C=C (a carbon-carbon double bond).
• This double bond makes alkenes much more reactive than alkanes.

Addition reactions

• Alkenes typically react by addition.
• The carbon-carbon double bond opens up into a single bond, allowing a new atom or group to join each of the two carbon atoms.
• Two molecules combine to form one saturated product:
  • alkene + X-Y → saturated product
• The X joins one carbon atom and the Y joins the other.
• The product is saturated because all the carbon-carbon bonds are single bonds.

The reactions of alkenes

• Alkenes have the functional group C=C, the carbon-carbon double bond, which decides how they react.
• The double bond can open up into a single bond, leaving each carbon atom with a spare bond where another molecule joins on. This is an addition reaction.
• Only one product is formed and the product is saturated.

A halogen adds across the double bond

• A halogen molecule splits, the double bond opens, and one halogen atom joins each of the two carbon atoms that used to share it.
• The products are called dihaloalkanes.
• Bromine is the halogen usually used. With ethene:
  • C₂H₄ + Br₂ → C₂H₄Br₂

• Chlorine (forming C₂H₄Cl₂) and iodine (forming C₂H₄I₂) react in the same way.
• Longer alkenes react at their double bond in the same way (e.g. propene and bromine form C₃H₆Br₂).

Hydrogen adds across the double bond

• Hydrogen adds across the double bond over a nickel catalyst at about 60 °C.
• The hydrogen molecule splits, the double bond opens, and one hydrogen atom joins each carbon. This reaction is called hydrogenation.
• The product is the alkane with the same number of carbon atoms (e.g. ethene forms ethane).
• ethene + hydrogen → ethane
• C₂H₄ + H₂ → C₂H₆

Steam adds across the double bond

• Steam adds across the double bond when passed over an acid catalyst at about 300 °C.
• The water molecule splits into a hydrogen atom and an -OH group, which join across the double bond. This reaction is called hydration.
• The product is an alcohol. Ethene gives ethanol:
  • ethene + steam → ethanol
  • C₂H₄ + H₂O → C₂H₅OH

The bromine water test

Bromine water is an orange solution of bromine in water. Put a sample of an unknown hydrocarbon in a test tube, add a few drops of bromine water, and shake.

• With an alkane, nothing happens and the bromine water stays orange because there's no double bond for bromine to add across.
• With an alkene, the orange colour disappears, turning the mixture colourless, because bromine adds across the double bond.

The change must be described as orange to colourless, not "clear", because the starting solution is already clear.