Classifying Organic Reactions (Edexcel IGCSE Chemistry)

• The reactions of organic compounds can be classified into substitution, addition and 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:

CH₄   +   2O₂   →   CO₂   +   2H₂O

C₃H₈   +   5O₂   →   3CO₂   +   4H₂O

• This is termed complete combustion
  • If there is a limited supply of air / oxygen, the products are carbon monoxide and water:

CH₄   +   O₂   →   CO   +   2H₂O

• This is termed incomplete combustion