Addition Reactions of Alkenes
- The chemistry of the alkenes is determined by the C=C functional group.
- Since all members of the alkene homologous series contain the same functional group then they all react similarly.
- Alkenes mainly undergo addition reactions in which atoms of a simple molecule add across the C=C double bond.
- The carbon carbon double bond opens up, forming a single bond between the carbons allowing for two more atoms to bond, one on each carbon.
Diagram showing the general equation for the addition reaction of alkenes
- Hydrogen, water and the halogens can take part in these reactions.
- Alkenes undergo addition reactions with hydrogen in which an alkane is formed.
- These are hydrogenation reactions and occur at 150ºC using a nickel catalyst.
- Hydrogenation reactions are used to change vegetable oils into margarine to be sold in supermarkets.
Hydrogen atoms add across the C=C in the hydrogenation of ethene to produce an alkane
- Alkenes also undergo addition reactions with steam in which an alcohol is formed. Since water is being added to the molecule it is also called a hydration reaction.
- The reaction is very important industrially for the production of alcohols and it occurs using the following conditions:
- Temperature of around 330ºC.
- Pressure of 60 – 70 atm.
- Concentrated phosphoric acid catalyst.
- When the reaction is complete, the reaction chamber holds unreacted ethene, ethanol and water.
- The contents are transferred to a condenser where ethene is separated easily as it has a much lower boiling point than ethanol and water:
- Ethanol: 78oC
- Ethene: -103oC
- Water: 100oC
- The ethanol and water are separated afterwards by fractional distillation.
A water molecule adds across the C=C in the hydration of ethene to produce ethanol
- The halogens also participate in addition reactions with alkenes.
- The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms involved in the C=C double bond.
Chlorine atoms add across the C=C in the addition reaction of ethene and chlorine
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Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.