The Allylic Bromination Niche: A Special Free Radical Reaction
Master selective bromination at allylic position using N-Bromosuccinimide (NBS) - A crucial tool for JEE Organic Chemistry.
Why Allylic Bromination is Special
Allylic bromination using N-Bromosuccinimide (NBS) is one of the most selective free radical reactions in organic chemistry. Unlike regular bromination that adds across double bonds, NBS specifically targets the allylic position - the carbon adjacent to the double bond.
🎯 JEE Importance
This reaction appears in 1-2 questions per JEE paper, testing understanding of free radical mechanisms, stability of intermediates, and selective organic transformations.
🚀 Quick Navigation
1. Understanding N-Bromosuccinimide (NBS)
The Special Brominating Agent
NBS Structure
N-Bromosuccinimide
A cyclic bromoamide that provides
low concentration of Br₂
Key Features
- Provides low, constant concentration of Br₂
- Prevents addition reaction to double bond
- Facilitates selective free radical substitution
- Works under mild conditions (light or peroxide)
Why Not Use Br₂ Directly?
| Feature | Br₂ (Direct) | NBS |
|---|---|---|
| Reaction Type | Electrophilic Addition | Free Radical Substitution |
| Position | Across double bond | Allylic position only |
| Selectivity | Low | High (95%+) |
| Br₂ Concentration | High | Low and constant |
2. The Free Radical Mechanism
Step-by-Step Mechanism
Step 1: Initiation - Br₂ Generation
NBS reacts with traces of HBr to generate low concentration of Br₂:
NBS + HBr → Succinimide + Br₂
Light or peroxide generates Br• radicals:
Br₂ → 2Br• (under light/Δ)
Step 2: Hydrogen Abstraction
Br• radical abstracts hydrogen from allylic position:
CH₃-CH=CH₂ + Br• → •CH₂-CH=CH₂ + HBr
Forms resonance-stabilized allylic radical
Step 3: Resonance Stabilization
The allylic radical is stabilized by resonance:
•CH₂-CH=CH₂ ↔ CH₂=CH-CH₂•
This resonance stabilization makes the reaction selective for allylic position
Step 4: Bromine Atom Transfer
The allylic radical reacts with Br₂ to form product and regenerate Br•:
•CH₂-CH=CH₂ + Br₂ → BrCH₂-CH=CH₂ + Br•
The chain reaction continues until all NBS is consumed
Complete Reaction Example
Propene + NBS → 3-Bromopropene
3. Understanding Selectivity & Regiochemistry
Why Allylic Position?
Resonance Stabilization
Allylic radicals are stabilized by resonance, making them more stable than other radical positions:
- Allylic radical: Resonance stabilized
- Primary radical: Less stable
- Vinylic radical: Less stable (sp² hybridized)
Bond Dissociation Energies
| Bond Type | Energy (kcal/mol) |
|---|---|
| Allylic C-H | ~88 |
| Primary C-H | ~101 |
| Vinylic C-H | ~111 |
Regiochemistry in Unsymmetrical Alkenes
Example: 1-Butene with NBS
Two possible allylic positions lead to two products:
The more substituted allylic position is preferred due to greater radical stability
💡 Key Selectivity Rules
- Allylic > Benzylic > Tertiary > Secondary > Primary > Methyl > Vinylic
- More substituted allylic positions are preferred
- Symmetrical alkenes give single product
- Unsymmetrical alkenes give mixture (follow radical stability)
4. Reaction Conditions & Limitations
Optimal Conditions
Required Conditions
- Initiator: Light (hv) or Peroxides (ROOR)
- Solvent: CCl₄ (inert, non-nucleophilic)
- Temperature: Room temperature or reflux
- Atmosphere: Often under N₂ to prevent O₂ inhibition
What Doesn't Work
- Alkanes without allylic positions
- Compounds with only vinylic hydrogens
- In presence of strong nucleophiles
- Under ionic conditions (gives addition)
Common Mistakes to Avoid
Leads to addition reaction instead of substitution
Reaction won't start without light or peroxide
Polar solvents can lead to side reactions
5. Practice Problems
Test Your Understanding
Problem 1: Predict the product when cyclohexene reacts with NBS in CCl₄ under light.
Problem 2: Why does NBS give substitution while Br₂ gives addition with alkenes?
Problem 3: Predict all possible products when 2-pentene reacts with NBS.
Problem 4: Explain why vinylic bromination doesn't occur with NBS.
JEE Tip
Always draw the resonance structures of allylic radicals - this is frequently asked in JEE to test fundamental understanding.
📋 Quick Reference Guide
Key Features
- Reagent: N-Bromosuccinimide (NBS)
- Mechanism: Free radical substitution
- Position: Allylic carbons only
- Conditions: Light/peroxide, CCl₄
- Selectivity: 95%+ for allylic position
Mechanism Steps
- Br₂ generation from NBS + HBr
- Initiation: Br• formation
- Propagation: H-abstraction → allylic radical
- Resonance stabilization
- Product formation + chain continuation
Common Substrates
→ 3-Bromopropene
→ 3-Bromocyclohexene
→ Benzyl bromide
→ 1-Bromo-2-butene
🎯 JEE Exam Strategy
Always start by writing the free radical mechanism - it helps predict products correctly.
Quickly mark all allylic carbons before predicting products.
Draw resonance structures for allylic radicals - often asked directly.
Mention "light or peroxide" and "CCl₄ solvent" for complete answers.
Mastered Allylic Bromination?
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