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Properties of Alkenes - Addition Reactions

Chemistry: Cambridge International GCSE Supplement, topic 11.5.7

Properties of Alkenes: Addition Reactions

Alkenes react in addition reactions with bromine, hydrogen, and steam, demonstrating specific properties and the reaction products.

Introduction to alkenes and addition reactions

Introduction to alkenes and addition reactions

  • Alkenes are hydrocarbons with at least one carbon-carbon double bond.
  • Alkenes undergo addition reactions that break the double bond.
  • Addition reactions add atoms to the carbon atoms originally in the double bond.
  • These reactions help us study alkene properties and identify them.
  • In addition reactions, only one product is formed.
Addition reaction with bromine or aqueous bromine

Addition reaction with bromine or aqueous bromine

  • When bromine (Br2) reacts with an alkene, the double bond breaks.
  • The bromine atoms add across the double bond.
  • This reaction changes the solution colour from orange-brown to colourless.
  • The product is a dibromo alkane.
  • Example: Ethene (C2H4) + Br2 → 1,2-dibromoethane (C2H4Br2).
Addition of hydrogen with nickel catalyst

Addition of hydrogen with nickel catalyst

  • Alkenes react with hydrogen (H2) in the presence of a nickel (Ni) catalyst.
  • The double bond breaks, and hydrogen atoms add to the carbons.
  • This process is called hydrogenation, turning alkenes into alkanes.
  • Example: Ethene + H2 → Ethane (C2H6).
  • Catalysts speed up reactions without changing themselves.
Addition of steam with acid catalyst

Addition of steam with acid catalyst

  • Steam (H2O) adds to alkenes in the presence of an acid catalyst like sulfuric acid (H2SO4).
  • The double bond breaks, forming an alcohol.
  • This process is called hydration.
  • Example: Ethene + steam → ethanol (C2H5OH).
  • Acid catalyst helps the reaction proceed faster.
Structural and displayed formulae of products

Structural and displayed formulae of products

  • For addition with bromine: from C=C to C-C with Br atoms attached.
  • For addition with hydrogen: from C=C to C-C with H atoms added.
  • For addition with steam: from C=C to C-C with OH and H atoms added.
  • Drawing structures helps visualise where new atoms join the chain.
  • Knowing these formulas shows how alkenes change during reactions.

Test yourself on Properties of Alkenes - Addition Reactions

  • What happens to the carbon-carbon double bond in an alkene during an addition reaction?

  • What is the role of a catalyst in a chemical reaction?

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