2.7.1 Types of Reaction

• Reactions can be classified as:
  • Addition
  • Elimination
  • Substitution
  • Oxidation
  • Reduction
  • Hydrolysis
  • Polymerisation

Addition reactions

• These involve two reactants combining to form one product
• The most common example of an addition reaction involves alkenes, e.g.

C₂H₄ + H₂ → C₂H₆ 

C₂H₄ + Br₂ → C₂H₄Br₂

C₂H₄ + H₂O → C₂H₅OH

Elimination reactions

• These are usually identified by one or two reactants forming more products
  • The products often include an alkene and a smaller molecule that has been eliminated from the reactant, e.g.

CH₃CH₂Br + OH^- C₂H₄ + H₂O + Br^–

Substitution reactions

• An atom or group of atoms in a compounds is replaced by another atom or group of atoms, e.g. 

CH₃CH₂Br + OH^- → C₂H₅OH + Br^–

Oxidation reactions

• This can involve the loss of hydrogen or the gain of oxygen
• A reactant is oxidised by another chemical species, usually an inorganic reagent, e.g.

CH₃CHO + [O] → CH₃COOH

• In this example, the oxidising agent [O] could be acidified potassium dichromate(VI) solution, Fehling's solution or Tollens' reagent

Reduction reactions

• This can involve the gain of hydrogen or the loss of oxygen
• A reactant is reduced by another chemical species
  • This can often be completed using a catalyst and a small reactant molecule such as H₂ or HCl, e.g.

CH₃CHO + [H] → CH₃CH₂OH

• In this example, the oxidising agent [H] could be lithium aluminium hydride

Hydrolysis reactions

• The name can be misleading as hydrolysis suggests that water is being used to split (or lyse) a compound
• Hydrolysis reactions are a specific reaction with water, e.g.

CH₃Cl + H₂O → CH₃OH + HCl

Polymerisation reactions

• At AS level, you should know about addition polymerisation
• This is where the carbon-carbon double bond of an alkene monomer breaks open and forms a polymer chain, e.g. 

Addition polymerisation of one alkene monomer is polymerised, a (poly)alkene is formed