Chapter 23- Alkenes

• Alkenes are unsaturated hydrocarbons with double bonds between two carbon atoms.

• The general formula is CnH2n

  

• Isomers of alkenes can be formed by branching them or changing the position of the double bond.

• Catalytic cracking is used to manufacture alkenes from alkanes. It can also be used to manufacture hydrogen.

• An alkane can be broken in three ways using cracking: Making one alkane and one alkene, and one hydrogen, making two alkenes and hydrogen, making an alkane.

• Alkenes can also be combusted.

ADDITION REACTIONS OF ALKENES

• Alkenes undergo addition reactions, i.e. reactions where only one product is formed. Double bonds are broken into single bonds.
• HYDROGENATION: In presence of the catalyst nickel and 200 degrees celsius temperature, alkenes react with hydrogen to form alkanes.
• Used in manufacture of margarine from vegetable oil.
• BROMINATION: when bromine is added to an alkene, it immediately decolourises as the double bonds are broken and bromine atoms join the alkene.
• Used in the test for saturation as bromine water decolourises in unsaturated substances only.
• HYDRATION: In the presence of phosphoric (V) acid, 300 degrees celsius temperature and 60 atm pressure, alkenes react with steam to form alcohols.