Classifying Organic Reactions (Edexcel IGCSE Chemistry)

Spec point — understand how to classify reactions of organic compounds as substitution, addition and combustion knowledge of reaction mechanisms is not required

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

  • Example: The complete combustion of methane and propane:

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

  • Example: The incomplete combustion of methane:

CH₄+$\frac{3}{2}$O₂→ CO + 2H₂O