Addition Reactions (Cambridge O Level Chemistry)

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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-Addition-to-Ethene, IGCSE & GCSE Chemistry revision notesBromine 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 

Addition- Hydrogen to Ethene, IGCSE & GCSE Chemistry revision notesHydrogen 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

Addition- Water to Ethene, IGCSE & GCSE Chemistry revision notesA water molecule adds across the C=C in the hydration of ethene to produce ethanol

Examiner 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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Caroline

Author: Caroline

Expertise: Physics Lead

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.