Substitution Reaction – IGCSE Chemistry Definition
IGCSE Chemistry definition of substitution reaction: an atom replaces another atom in a molecule. Covers alkane substitution with halogens and exam tips for 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Substitution reactions are the characteristic reactions of saturated compounds (alkanes). The comparison with addition reactions (characteristic of unsaturated alkenes) is a classic exam question worth 2–3 marks.
The 0620 definition
A substitution reaction is a reaction in which an atom (or group of atoms) in a molecule is replaced by another atom (or group of atoms).
The key example: alkane + halogen
Methane reacts with chlorine in UV light:
CH₄ + Cl₂ → CH₃Cl + HCl
- One H atom in methane is replaced by a Cl atom
- Two products are formed (chloromethane + hydrogen chloride)
- UV light is the essential condition — it provides the energy to break the Cl−Cl bond
Further substitution can continue:
CH₃Cl + Cl₂ → CH₂Cl₂ + HCl (dichloromethane)
CH₂Cl₂ + Cl₂ → CHCl₃ + HCl (trichloromethane)
CHCl₃ + Cl₂ → CCl₄ + HCl (tetrachloromethane)
Substitution vs addition
| Feature | Substitution | Addition |
|---|---|---|
| Typical reactant | Saturated (alkane) | Unsaturated (alkene) |
| Number of products | Two | One |
| What happens | An atom is replaced | Atoms are added across C=C |
| Double bond? | No | Yes — it opens |
| Example | CH₄ + Cl₂ → CH₃Cl + HCl | CH₂=CH₂ + Cl₂ → CH₂ClCH₂Cl |
Why alkanes undergo substitution, not addition
Alkanes have no C=C double bond to open. All their C−C and C−H bonds are single bonds. The only way to react is to break a C−H bond and replace the hydrogen with something else — this is substitution.
Worked exam question
Ethane reacts with bromine in the presence of UV light. (a) Name the type of reaction. [1] (b) Write the equation for the reaction. [1] (c) State the essential condition. [1]
Mark scheme
(a) Substitution [1]
(b) C₂H₆ + Br₂ → C₂H₅Br + HBr [1]
(c) UV light / sunlight [1]
Common exam mistakes
- Forgetting to state “UV light” as the condition — without this, the reaction does not occur. Heat alone is not sufficient.
- Saying substitution produces one product — it produces two (the substituted organic compound and HCl/HBr).
- Calling the reaction between methane and chlorine an “addition” — methane is saturated, so it cannot undergo addition.
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