Addition Reactions (Cambridge (CIE) O Level Chemistry): Revision Note

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

C₂H₄ + Br₂ → C₂H₄Br₂

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 150 °C using a nickel catalyst

• Hydrogenation reactions are used to manufacture margarine from vegetable oils

  • Vegetable oils are polyunsaturated molecules which are partially hydrogenated to remove some of the C=C double bonds, allowing the chains to pack together more closely and raising the melting point so the oils become solid fats

C₂H₄ + H₂ → C₂H₆

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 300 °C

  • Pressure of 60 atm (6000 kPa)

  • Concentrated phosphoric acid catalyst

C₂H₄ + H₂O → C₂H₅OH

A water molecule adds across the C=C in the hydration of ethene to produce ethanol