Radical substitution is a key reaction mechanism in IB Chemistry Topic 10 (Organic Chemistry). It explains how alkanes, which are normally very unreactive, can undergo substitution reactions when exposed to halogens and ultraviolet light. Understanding this mechanism helps you interpret reaction pathways, predict organic products, and recognize chain reactions in exam questions.
What Is Radical Substitution?
Radical substitution is a reaction where a halogen atom replaces a hydrogen atom in an alkane through a chain reaction involving free radicals.
Breaking the term down:
- Radical = species with an unpaired electron
- Substitution = one atom or group replaces another
- Chain reaction = continuous process of radical generation and propagation
This mechanism is characteristic of alkanes, which cannot undergo addition reactions due to their lack of C=C double bonds.
Why Radical Substitution Happens
Alkanes are:
- Non-polar
- Saturated
- Generally unreactive
However, halogens (especially chlorine and bromine) become highly reactive under UV light.
UV light:
- Splits halogen molecules into radicals
- Starts a chain reaction
- Allows alkanes to react
Because radicals are extremely reactive, the reaction proceeds rapidly once initiated.
Conditions Required
Radical substitution requires:
- Halogen (Cl₂ or Br₂)
- UV light or sunlight
- Alkane (e.g., methane, ethane)
No reaction occurs between alkanes and halogens without UV light.
The Three Stages of Radical Substitution
IB Chemistry requires you to know the entire mechanism:
Initiation → Propagation → Termination
1. Initiation
This step creates radicals using UV light.
Example with chlorine:
Cl₂ → 2Cl·
(· = unpaired electron)
This is homolytic fission, where the bond breaks evenly and each atom takes one electron.
2. Propagation (Chain Reaction)
Propagation keeps the reaction going and forms the main products.
Step 1:
A chlorine radical attacks the alkane:
Cl· + CH₄ → HCl + CH₃·
A methyl radical forms.
Step 2:
The methyl radical reacts with another chlorine molecule:
CH₃· + Cl₂ → CH₃Cl + Cl·
Another chlorine radical is regenerated, sustaining the chain reaction.
This cycle repeats thousands of times.
3. Termination
Termination occurs when two radicals combine, removing radicals from the reaction.
Examples:
Cl· + Cl· → Cl₂
CH₃· + Cl· → CH₃Cl
CH₃· + CH₃· → C₂H₆
These reactions stop the chain process by pairing unpaired electrons.
Products of Radical Substitution
Multiple substitutions can occur because products can continue reacting.
For methane and chlorine:
- CH₃Cl
- CH₂Cl₂
- CHCl₃
- CCl₄
As a result, the reaction mixture often contains many different products unless conditions are carefully controlled.
Selectivity: Chlorine vs Bromine
Chlorine
- Very reactive
- Fast reactions
- Low selectivity
- Many by-products
Bromine
- Less reactive
- More selective
- Cleaner substitution
This difference is often tested in IB exam questions.
Why Radical Substitution Is Important in IB Chemistry
You must be able to:
- Write initiation, propagation, and termination steps
- Identify radicals
- Explain homolytic fission
- Predict products
- Understand why UV light is essential
These skills appear frequently in Paper 2 structured questions.
Common IB Misunderstandings
“Radicals are ions.”
Incorrect. Radicals have unpaired electrons, not charges.
“Alkanes react easily without UV.”
They do not. Their C–H bonds are too strong without radical formation.
“Only one product forms.”
Multiple substitution products are common.
“This is electrophilic substitution.”
No—radical substitution is a distinct mechanism.
FAQs
Why is UV light needed?
It provides energy to break the halogen bond via homolytic fission, forming radicals.
Can radical substitution happen with alkenes?
Yes, but alkenes usually undergo addition reactions instead because they are more reactive.
Why are multiple substitutions common?
Once the first hydrogen is replaced, the molecule can react again under the same radical-producing conditions.
Conclusion
Radical substitution is a chain reaction where halogens replace hydrogens in alkanes. It proceeds through initiation (radical formation), propagation (chain reaction), and termination (radical removal). This mechanism explains how unreactive alkanes can form halogenoalkanes under UV light and is essential knowledge for IB Chemistry students.
