Classifying Organic Reactions (Edexcel IGCSE Science (Double Award) (Modular)): Revision Note

Classifying organic reactions

• The reactions of organic compounds can be classified into

  • Substitution

  • Addition

  • Combustion

Substitution

• A substitution reaction takes place when one functional group is replaced by another

  • Example: Methane reacts with bromine under ultraviolet light

CH₄    +    Br₂       →            CH₃Br   +    HBr

    methane + bromine   →   bromomethane + hydrogen bromide

Addition

• An addition reaction takes place when two or more molecules combine to form a larger molecule with no other products

  • Example: Bromine will react with ethene and the bromine molecule will react and add across the double bond of the ethene

C₂H₄     +       Br₂         →         C₂H₄Br₂

ethene   +   bromine   →   dibromoethane

Combustion

• This is the scientific term for burning. In a combustion reaction, an organic substance reacts with oxygen to form carbon dioxide (or carbon monoxide if incomplete combustion) and water.

  • Example: Alkenes burn when heated in air of oxygen

• If there is an unlimited supply of air / oxygen, the products are carbon dioxide and water

• This is termed complete combustion

CH₄   +   2O₂   →   CO₂   +   2H₂O

C₃H₈   +   5O₂   →   3CO₂   +   4H₂O

• If there is a limited supply of air / oxygen, the products are carbon monoxide and water

  • This is termed incomplete combustion

CH₄   +   O₂   →   CO   +   2H₂O