Addition Reactions (Cambridge O Level Chemistry)
Revision Note
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Addition Reactions
- Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond
- The reaction between bromine and ethene is an example of an addition reaction
Bromine atoms add across the C=C in the addition reaction of ethene and bromine
- Alkenes also 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 manufacture margarine from vegetable oils
- Vegetable oils are polyunsaturated molecules which are partially hydrogenated to increase the Mr and turn the oils into solid fats
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 300ºC
- Pressure of 60 - 70 atm
- Concentrated phosphoric acid catalyst
A water molecule adds across the C=C in the hydration of ethene to produce ethanol
Exam Tip
You need to be able to draw the displayed formulae of the products of alkenes with water, hydrogen and bromine.
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