Addition Reactions (Cambridge O Level Chemistry)

• 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 M_r 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