3.10.3Organic Chemistry
Free Radical Substitution of Arenes
5 min read 5 parts Lesson 39 of 55
Section 1 of 5Side Chain or Ring
Side Chain or Ring
Conditions decide
- Reactions on an arene can take place on the ring or on an alkyl side chain, and the conditions decide which one occurs.
Ring substitution
- Chlorine with an AlCl₃ or FeCl₃ catalyst, in the dark, replaces a hydrogen on the benzene ring by an electrophilic mechanism.
Side-chain substitution
- Chlorine with UV light replaces a hydrogen on the CH₃ group by a free radical mechanism, with no catalyst required.
Why
- UV light supplies the energy to split Cl₂ into radicals, and the side chain behaves like an alkane.
| Feature | Ring substitution | Side-chain substitution |
|---|---|---|
| Conditions | AlCl₃, room temperature, dark | UV light, no catalyst |
| Species attacking | Cl⁺ electrophile | Cl• radical |
| Product with methylbenzene | chloromethylbenzenes on the ring | (chloromethyl)benzene |
Section 2 of 5The Chain Mechanism
1
Initiation: UV light breaks the Cl–Cl bond homolytically to give two chlorine radicals, Cl₂ → 2Cl•.
2
Propagation 1: a chlorine radical removes a hydrogen from the side chain, C₆H₅CH₃ + Cl• → C₆H₅CH₂• + HCl.
3
Propagation 2: the benzyl radical reacts with chlorine, C₆H₅CH₂• + Cl₂ → C₆H₅CH₂Cl + Cl•, and the chain continues.
4
Termination: two radicals combine, for example 2Cl• → Cl₂, or C₆H₅CH₂• + Cl• → C₆H₅CH₂Cl.
5
A termination step can also make a by-product, 2C₆H₅CH₂• → C₆H₅CH₂CH₂C₆H₅, which shows the chain can end in several ways.
Section 3 of 5Further Substitution
Mono-substitution
- Using a limited amount of chlorine gives mostly (chloromethyl)benzene, C₆H₅CH₂Cl.
Di-substitution
- With more chlorine, a second hydrogen is replaced to give (dichloromethyl)benzene, C₆H₅CHCl₂.
Tri-substitution
- A third replacement gives (trichloromethyl)benzene, C₆H₅CCl₃, so a mixture is usually formed.
Bromine
- Bromine under UV light reacts in the same way to form (bromomethyl)benzene and HBr.
Section 4 of 5Mechanism Type
Mechanism Type
Radical
- A species with an unpaired electron, which is very reactive and makes the chain reaction possible.
Half-headed arrows
- Fishhook arrows show the movement of one electron when bonds break homolytically.
Selectivity
- The benzene ring itself is not attacked by radicals, because the aromatic system is stable.
Section 5 of 5Exam Checklist & Tips
3.10.3 – Key Points for the Exam
- Free radical substitution A chain reaction where a hydrogen is replaced via radical intermediates.
- Initiation The step in which radicals are first formed, using UV light.
- Propagation Steps in which a radical reacts and another is formed, continuing the chain.
- Termination Steps in which two radicals combine and the chain ends.
Common Mistakes
- Using AlCl₃ for side-chain chlorination; UV light is the condition for the side chain.
- Showing the UV step with a full arrow instead of two half-headed arrows.
- Forgetting that the propagation steps regenerate a radical, so the chain keeps going.
- Saying substitution happens on the ring when UV light is stated.