Topic 4 · page 5 of 17

Types of organic reaction

Most organic reactions fall into five types. You classify a reaction by looking at what happens to the reactants, and a single reaction can belong to more than one type.

9 min read4.6

Five types of reaction 4.6

You are not expected to memorise hundreds of reactions. You are expected to say what kind of change is happening. There are five kinds.

Type What happens Example
Addition two molecules join to make one product CH₂=CH₂ + Br₂ → CH₂BrCH₂Br
Substitution an atom or group is replaced by another CH₄ + Cl₂ → CH₃Cl + HCl
Oxidation gain of oxygen, or loss of hydrogen CH₄ + 2O₂ → CO₂ + 2H₂O
Reduction gain of hydrogen, or loss of oxygen CH₂=CH₂ + H₂ → CH₃CH₃
Polymerisation many small monomer molecules join to make one polymer n CH₂=CH₂ → –[CH₂–CH₂]ₙ–

Later in the course you will define oxidation and reduction in terms of electrons and oxidation numbers. For organic compounds, the changes in oxygen and hydrogen are the quickest test, and they are the ones to use here.

How to decide

MethodClassifying a reaction

  1. Count the products. One product from two reactants points to addition, or to polymerisation if there are very many monomer units.
  2. Two products, with an atom or group swapped for another, is substitution.
  3. Look for oxygen or hydrogen being gained or lost. Gain of O or loss of H is oxidation. Gain of H or loss of O is reduction.
  4. If more than one fits, name all that apply.

Worked exampleClassify C₂H₆ + Br₂ → C₂H₅Br + HBr

  1. There are two products, so it is not addition.
  2. One H atom of ethane has been replaced by a Br atom, and the H has gone to form HBr.
  3. This is substitution.

Check yourself

Classify each reaction as addition, substitution, oxidation, reduction or polymerisation. Name all that apply.

  1. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
  2. CH₂=CH₂ + HBr → CH₃CH₂Br
  3. CH₄ + Br₂ → CH₃Br + HBr
  4. n CH₂=CHCH₃ → –[CH₂–CH(CH₃)]ₙ–
  5. CH₂=CHCH₃ + H₂ → CH₃CH₂CH₃
  6. State what is meant by a monomer.

Answers cover these until you have tried the questions

  1. Oxidation. The propane gains oxygen (it burns to carbon dioxide and water).
  2. Addition. Two molecules join to make one product, and nothing is lost.
  3. Substitution. A hydrogen atom of methane is replaced by a bromine atom, and HBr is also formed.
  4. Polymerisation. Many propene molecules join to form one long chain.
  5. Addition and reduction. Two molecules make one product, and the organic molecule gains hydrogen.
  6. A monomer is a small molecule that joins with many others of the same kind to make a polymer.