Substitution Reaction
Summary: A substitution reaction is a reaction in which one atom or group of atoms in a molecule is replaced by another atom or group. At IGCSE, this applies to alkanes reacting with halogens (e.g. chlorine) in the presence of UV light (photochemical reaction). Tags: igcse chemistry definition organic-chem Created: 2026-07-14 Last Updated: 2026-07-23
A substitution reaction is a chemical reaction where one atom or group of atoms is replaced by another atom or group of atoms.
At IGCSE, substitution reactions are studied in the context of alkanes reacting with halogens (specifically chlorine or bromine), where a hydrogen atom on the alkane is replaced by a halogen atom.
Substitution of Alkanes with Chlorine
General Equation
CH₄ + Cl₂ →(UV light)→ CH₃Cl + HCl
Methane reacts with chlorine in the presence of ultraviolet (UV) light to form chloromethane and hydrogen chloride.
The Reaction Is Photochemical
- Requires UV light to initiate
- UV light provides the activation energy (Ea)
- UV light breaks the Cl—Cl bond to produce reactive chlorine atoms (free radicals)
- The reaction is a chain reaction — once started, it continues
Further Substitution
If excess chlorine is present, further substitution can occur:
- CH₃Cl + Cl₂ → CH₂Cl₂ + HCl (dichloromethane)
- CH₂Cl₂ + Cl₂ → CHCl₃ + HCl (trichloromethane)
- CHCl₃ + Cl₂ → CCl₄ + HCl (tetrachloromethane)
IGCSE only requires monosubstitution (replacement of one hydrogen).
Substitution vs Addition
| Feature | Substitution | Addition |
|---|---|---|
| What happens | One atom replaces another | Atoms add across a double bond |
| Substrate | Alkanes (saturated, C—C only) | Alkenes (unsaturated, C=C) |
| Products | TWO products (organic + HX) | ONE product only |
| Light required? | Yes — UV light | No |
Key Facts
- Substitution = one atom/group replaces another
- Alkanes undergo substitution with chlorine or bromine
- Requires UV light (photochemical reaction)
- Products: haloalkane + hydrogen halide (two products)
- UV light provides activation energy by breaking the halogen bond
- Only monosubstitution is required for IGCSE
See Also
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.