The Hook: Why Doesn’t Chlorobenzene React Like Chloroethane?
DDT (dichlorodiphenyltrichloroethane), once the world’s most famous insecticide, contains aromatic chlorine atoms. These C-Cl bonds are so stable that DDT persists in the environment for decades! In contrast, alkyl chlorides are reactive and decompose quickly.
Here’s the JEE question: Why is chlorobenzene 1000 times less reactive than chloroethane in nucleophilic substitution? And why does chlorobenzene direct incoming electrophiles to ortho and para positions, even though it’s a deactivating group?
The Core Concept
What are Haloarenes?
Haloarenes (Aryl Halides) are compounds where halogen is directly attached to benzene ring.
General structure:
$$\boxed{\text{Ar-X}}$$where Ar = aromatic ring, X = F, Cl, Br, I
Examples:
- Chlorobenzene: C₆H₅Cl
- Bromobenzene: C₆H₅Br
- p-Dichlorobenzene: Cl-C₆H₄-Cl
Key Difference:
Haloalkanes: X attached to sp³ carbon
- Example: CH₃CH₂Cl
- Reactive in nucleophilic substitution
Haloarenes: X attached to sp² carbon of benzene
- Example: C₆H₅Cl
- Unreactive in nucleophilic substitution (under normal conditions)
Why the difference?
- Resonance: C-X bond has partial double bond character
- Hybridization: sp² carbon holds electrons more tightly than sp³
- Stability: Breaking C-X disrupts aromatic resonance
Why Haloarenes are Unreactive
Reason 1: Resonance Stabilization
Lone pair on halogen delocalizes into benzene ring.
Resonance structures:
Cl Cl⁺ Cl⁺ Cl⁺
| | | |
⚬ ↔ ⚬ ↔ ⚬ ↔ ⚬
(-) (-) (-)
Consequence:
- C-Cl bond has partial double bond character
- Bond length: 169 pm (vs 177 pm in alkyl chlorides)
- Stronger and shorter → harder to break
Reason 2: sp² Hybridization
In haloarenes:
- Carbon is sp² hybridized
- Higher s-character (33%) vs sp³ (25%)
- Electrons held more tightly
- C-X bond is stronger
Reason 3: Stability of Phenyl Cation
If nucleophilic substitution occurred:
$$\text{C}_6\text{H}_5\text{-Cl} \rightarrow \text{C}_6\text{H}_5^+ + \text{Cl}^-$$Phenyl cation (C₆H₅⁺):
- Extremely unstable
- Cannot be stabilized by resonance
- Positive charge on sp² carbon (not favorable)
- Does NOT form under normal conditions
“RIP: Resonance, Inert, Phenyl cation”
Resonance makes C-X bond stronger Inert sp² carbon (high s-character) Phenyl cation too unstable
JEE Tip: This explains why chlorobenzene doesn’t react with aq. NaOH, but alkyl chlorides do!
Preparation of Haloarenes
Method 1: From Benzene (Electrophilic Halogenation)
Direct halogenation:
$$\boxed{\text{C}_6\text{H}_6 + \text{X}_2 \xrightarrow{\text{Lewis acid}} \text{C}_6\text{H}_5\text{X} + \text{HX}}$$Catalyst required:
- For Cl₂: FeCl₃ or AlCl₃
- For Br₂: FeBr₃ or AlBr₃
- For I₂: HNO₃ (oxidizing agent)
Mechanism: Electrophilic aromatic substitution
Step 1: Generation of electrophile
$$\text{Cl}_2 + \text{FeCl}_3 \rightarrow \text{Cl}^+ + [\text{FeCl}_4]^-$$Step 2: Electrophilic attack
Cl⁺
↓
⚬ → ⚬⁺ → ⚬
H -H⁺ Cl
(σ-complex)
Example:
$$\text{C}_6\text{H}_6 + \text{Cl}_2 \xrightarrow{\text{FeCl}_3} \text{C}_6\text{H}_5\text{Cl} + \text{HCl}$$Method 2: Sandmeyer Reaction (Most Important for JEE)
From diazonium salts:
$$\boxed{\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{CuX} \rightarrow \text{C}_6\text{H}_5\text{X} + \text{N}_2 + \text{CuCl}}$$Works for: X = Cl, Br
Example:
$$\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{CuCl/HCl} \rightarrow \text{C}_6\text{H}_5\text{Cl} + \text{N}_2 \uparrow$$Advantages:
- Excellent yields
- Specific substitution position
- Works when direct halogenation would give wrong isomer
Method 3: Gattermann Reaction
Alternative to Sandmeyer:
$$\boxed{\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{HX/Cu} \rightarrow \text{C}_6\text{H}_5\text{X} + \text{N}_2}$$Difference from Sandmeyer:
- Uses HX instead of CuX
- Slightly lower yields
- Cheaper reagents
Method 4: For Iodobenzene
Iodine doesn’t work with Lewis acids (weak electrophile)
Method A: From diazonium salt + KI
$$\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{KI} \rightarrow \text{C}_6\text{H}_5\text{I} + \text{N}_2 + \text{KCl}$$Method B: Oxidative iodination
$$\text{C}_6\text{H}_6 + \text{I}_2 \xrightarrow{\text{HNO}_3} \text{C}_6\text{H}_5\text{I}$$HNO₃ oxidizes I₂ to I⁺ (electrophile)
Method 5: For Fluorobenzene
Balz-Schiemann Reaction:
$$\boxed{\text{C}_6\text{H}_5\text{N}_2^+\text{BF}_4^- \xrightarrow{\Delta} \text{C}_6\text{H}_5\text{F} + \text{N}_2 + \text{BF}_3}$$Unique feature: Diazonium fluoroborate is dry and isolable
Q: How would you prepare m-chloronitrobenzene from benzene?
Analysis:
- Need meta-substitution
- Direct chlorination of nitrobenzene gives meta (NO₂ is m-director)
Correct sequence:
Step 1: Nitration
$$\text{C}_6\text{H}_6 \xrightarrow{\text{HNO}_3/\text{H}_2\text{SO}_4} \text{C}_6\text{H}_5\text{NO}_2$$Step 2: Chlorination
$$\text{C}_6\text{H}_5\text{NO}_2 + \text{Cl}_2 \xrightarrow{\text{FeCl}_3} m\text{-ClC}_6\text{H}_4\text{NO}_2$$Why this order?
- NO₂ is meta-directing
- Chlorination after nitration gives meta-product
Wrong order: Chlorination first would give o/p mixture, then nitration gives mixture!
JEE Tip: Order of substitution matters for getting correct isomer!
Reactions of Haloarenes
Type 1: Nucleophilic Substitution (Very Difficult)
Under normal conditions: Haloarenes DO NOT undergo nucleophilic substitution.
Harsh conditions required:
- Very high temperature (300-400°C)
- Very high pressure
- Strong base/nucleophile
Example (industrial):
$$\text{C}_6\text{H}_5\text{Cl} + \text{NaOH} \xrightarrow{623\text{ K, 300 atm}} \text{C}_6\text{H}_5\text{OH} + \text{NaCl}$$Activated Nucleophilic Aromatic Substitution (SNAr)
When haloarene has electron-withdrawing groups at ortho/para positions:
Mechanism: Addition-Elimination (not SN1 or SN2!)
Example: 2,4-dinitrochlorobenzene
NO₂ NO₂ NO₂
| | |
Cl—⚬—NO₂ + OH⁻ → Cl—⚬—NO₂ → HO—⚬—NO₂ + Cl⁻
|
OH
(Meisenheimer
complex)
Conditions for SNAr:
- Electron-withdrawing groups (NO₂, CN, COR) at o/p positions
- Groups stabilize negative charge in intermediate
- Room temperature reaction possible!
Electron-withdrawing groups activate SNAr:
Order of activation:
$$\boxed{\text{-NO}_2 > \text{-CN} > \text{-COCH}_3 > \text{-CHO} > \text{-COOH}}$$Position matters:
- o/p to halogen → highly activated
- meta to halogen → less effective
Most reactive: 2,4,6-trinitrochlorobenzene (picryl chloride)
- Can react with water at room temperature!
Example:
- 2,4-dinitrochlorobenzene: reacts easily
- 3,5-dinitrochlorobenzene: much slower (meta positions)
- Chlorobenzene: no reaction under normal conditions
JEE Strategy: Count NO₂ groups at o/p positions to predict reactivity!
Type 2: Electrophilic Substitution
Haloarenes undergo typical electrophilic aromatic substitution reactions.
Halogen effect:
- Deactivating: Withdraws electrons by inductive effect
- ortho/para directing: Donates electrons by resonance
Nitration
$$\text{C}_6\text{H}_5\text{Cl} + \text{HNO}_3 \xrightarrow{\text{H}_2\text{SO}_4} o\text{-ClC}_6\text{H}_4\text{NO}_2 + p\text{-ClC}_6\text{H}_4\text{NO}_2$$Products: ortho + para mixture (para usually major)
Sulfonation
$$\text{C}_6\text{H}_5\text{Br} + \text{H}_2\text{SO}_4 \xrightarrow{\text{heat}} o/p\text{-BrC}_6\text{H}_4\text{SO}_3\text{H}$$Friedel-Crafts Alkylation
$$\text{C}_6\text{H}_5\text{Cl} + \text{CH}_3\text{Cl} \xrightarrow{\text{AlCl}_3} o/p\text{-ClC}_6\text{H}_4\text{CH}_3$$Note: Slower than benzene (deactivating effect)
Friedel-Crafts Acylation
$$\text{C}_6\text{H}_5\text{Br} + \text{CH}_3\text{COCl} \xrightarrow{\text{AlCl}_3} o/p\text{-BrC}_6\text{H}_4\text{COCH}_3$$Common mistake: “Cl is electron-withdrawing, so it’s meta-directing”
Correct: Halogen is ortho/para directing BUT deactivating
Why ortho/para?
- Resonance effect (electron donation by lone pair) > Inductive effect
- Lone pair on X can donate to ortho/para positions
Why deactivating?
- Inductive withdrawal of electrons from ring
- Makes ring less nucleophilic overall
- Reaction is slower than benzene
Memory: “Halogens are Odd: Ortho/para but Deactivating”
Type 3: Reduction
With Ni/H₂:
$$\text{C}_6\text{H}_5\text{Cl} + \text{H}_2 \xrightarrow{\text{Ni, } \Delta} \text{C}_6\text{H}_6 + \text{HCl}$$Catalytic hydrogenolysis: Removes halogen
Type 4: Reaction with Metals
Wurtz-Fittig Reaction:
$$\text{C}_6\text{H}_5\text{Br} + \text{CH}_3\text{Br} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{C}_6\text{H}_5\text{-CH}_3 + 2\text{NaBr}$$Fittig Reaction:
$$2\text{C}_6\text{H}_5\text{Br} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{C}_6\text{H}_5\text{-C}_6\text{H}_5 + 2\text{NaBr}$$Product: Biphenyl
Formation of Grignard Reagent:
$$\text{C}_6\text{H}_5\text{Br} + \text{Mg} \xrightarrow{\text{dry ether}} \text{C}_6\text{H}_5\text{MgBr}$$Phenyl magnesium bromide - useful in synthesis
Physical Properties
1. Physical State
- Chlorobenzene, bromobenzene: colorless liquids
- Iodobenzene: colorless to brown liquid
- p-Dichlorobenzene: white crystals (mothballs!)
2. Odor
- Characteristic aromatic odor
- p-Dichlorobenzene: distinctive mothball smell
3. Solubility
- Insoluble in water (non-polar)
- Soluble in organic solvents (benzene, ether, alcohol)
4. Boiling Points
- Higher than corresponding alkyl halides
- Due to stronger C-X bond and aromatic π-π interactions
| Compound | BP (°C) |
|---|---|
| Fluorobenzene | 85 |
| Chlorobenzene | 132 |
| Bromobenzene | 156 |
| Iodobenzene | 188 |
5. Density
- Denser than water
- Chlorobenzene: 1.11 g/mL
Uses and Applications
Chlorobenzene
- Solvent in organic synthesis
- Intermediate for phenol, aniline production
- DDT synthesis (insecticide production)
Bromobenzene
- Grignard reagent preparation
- Pharmaceutical intermediate
Iodobenzene
- Organic synthesis - iodine source
- Reagent in organometallic chemistry
p-Dichlorobenzene
- Mothballs and air fresheners
- Deodorant blocks
- Insecticide
Common Mistakes to Avoid
Wrong: “Chlorobenzene + NaOH → Phenol (like alkyl halides → alcohols)”
Correct: Haloarenes do NOT undergo simple nucleophilic substitution
Exception: With activating groups (NO₂ at o/p) or very harsh conditions
JEE Tip: Always note if halogen is on aromatic ring or aliphatic chain!
Interactive Demo: Visualize Haloarene Resonance Structures
See how halogen lone pairs delocalize into the aromatic ring.
Wrong: “Cl deactivates, so it’s meta-directing”
Correct: Halogens are ortho/para directing (despite being deactivating)
Reason: Resonance donation > Inductive withdrawal for directing effect
JEE Rule: Only -NO₂, -CN, -SO₃H, -COOH, -CHO, -COR are meta-directing
Wrong: Starting synthesis without planning substitution sequence
Correct: Use directing effects strategically
Example: To make m-bromoaniline:
- Wrong: Br first (o/p director) → NH₂ second gives o/p-isomers
- Correct: NO₂ first (m-director) → Br second → reduce NO₂ to NH₂
JEE Strategy: Plan backwards from target molecule!
Practice Problems
Level 1: Foundation (NCERT)
Q: Why is chlorobenzene less reactive than chloroethane toward nucleophilic substitution?
Answer:
Three reasons:
Resonance: Lone pair on Cl delocalizes into benzene ring
- C-Cl bond has partial double bond character
- Stronger and harder to break
Hybridization: sp² carbon (vs sp³ in chloroethane)
- Higher s-character
- Holds electrons more tightly
Phenyl cation instability: C₆H₅⁺ is extremely unstable
- Cannot form under normal conditions
Conclusion: All three factors make C-Cl bond unreactive in chlorobenzene.
Q: How will you prepare chlorobenzene from benzene?
Solution:
Method: Direct chlorination
$$\text{C}_6\text{H}_6 + \text{Cl}_2 \xrightarrow{\text{FeCl}_3} \text{C}_6\text{H}_5\text{Cl} + \text{HCl}$$Mechanism: Electrophilic aromatic substitution
- FeCl₃ generates Cl⁺ electrophile
- Cl⁺ attacks benzene ring
- H⁺ is eliminated
Level 2: JEE Main
Q: Arrange in order of increasing reactivity toward nucleophilic substitution: (a) Chlorobenzene (b) 2,4-Dinitrochlorobenzene (c) 2,4,6-Trinitrochlorobenzene
Solution:
Order: (a) < (b) < (c)
Reasoning:
(a) Chlorobenzene: No activating groups
- Extremely unreactive (needs 300°C, high pressure)
(b) 2,4-Dinitrochlorobenzene: Two NO₂ at o and p
- Moderately reactive (room temperature with strong base)
(c) 2,4,6-Trinitrochlorobenzene: Three NO₂ (all o/p)
- Very reactive (reacts with water!)
Key: More NO₂ groups at o/p positions → higher reactivity
Q: What are the major products when chlorobenzene undergoes nitration?
Solution:
$$\text{C}_6\text{H}_5\text{Cl} + \text{HNO}_3 \xrightarrow{\text{H}_2\text{SO}_4} ?$$Products:
- o-Chloronitrobenzene (ortho)
- p-Chloronitrobenzene (para) - major
Ratio: ~30% ortho, 70% para (para favored due to less steric hindrance)
No meta product - Cl is o/p director!
Level 3: JEE Advanced
Q: How will you prepare m-chloroaniline from benzene?
Solution:
Strategy: Need meta-substitution, so use meta-director first
Step 1: Nitration
$$\text{C}_6\text{H}_6 \xrightarrow{\text{HNO}_3/\text{H}_2\text{SO}_4} \text{C}_6\text{H}_5\text{NO}_2$$Step 2: Chlorination (NO₂ is meta-directing)
$$\text{C}_6\text{H}_5\text{NO}_2 + \text{Cl}_2 \xrightarrow{\text{FeCl}_3} m\text{-ClC}_6\text{H}_4\text{NO}_2$$Step 3: Reduction of NO₂ to NH₂
$$m\text{-ClC}_6\text{H}_4\text{NO}_2 \xrightarrow{\text{Sn/HCl}} m\text{-ClC}_6\text{H}_4\text{NH}_2$$Answer: Three-step synthesis
Why this order?
- NH₂ is o/p director (would give wrong isomer if done first)
- NO₂ is meta director (gives correct position)
- Reduce NO₂ to NH₂ at the end
JEE Tip: For meta-substitution, use NO₂, CN, or COOH as temporary director!
Q: Explain why 2,4-dinitrochlorobenzene reacts with NaOH at room temperature but chlorobenzene doesn’t.
Solution:
2,4-Dinitrochlorobenzene: Undergoes SNAr mechanism
Mechanism:
Step 1: Nucleophilic addition (rate-determining)
NO₂ NO₂
| |
Cl—⚬—NO₂ + OH⁻ → Cl—⚬—NO₂
|
OH⁻
(Meisenheimer
complex)
Step 2: Elimination of Cl⁻
NO₂ NO₂
| |
Cl—⚬—NO₂ → HO—⚬—NO₂ + Cl⁻
|
OH⁻
Why this works:
- NO₂ groups are electron-withdrawing
- Stabilize negative charge in intermediate by resonance
- Lower activation energy
Chlorobenzene:
- No electron-withdrawing groups
- Cannot stabilize negative intermediate
- Phenyl cation too unstable
- Needs extreme conditions (300°C, 300 atm)
Conclusion: Activating groups make ALL the difference!
Quick Revision Box
| Topic | Key Points | JEE Rule |
|---|---|---|
| Structure | X directly on benzene | sp² C-X bond |
| Unreactive | Resonance + sp² + unstable C⁺ | 1000× slower than alkyl |
| Preparation | Sandmeyer (best), direct halogenation | From diazonium salt |
| SNAr | Needs NO₂ at o/p | Addition-elimination |
| Directing | ortho/para but deactivating | Resonance > Inductive |
| Reduction | Ni/H₂ removes X | Gives benzene |
| Wurtz-Fittig | Ar-X + R-X + Na | Gives Ar-R |
| Uses | Solvents, intermediates, DDT | p-DCB = mothballs |
Connection to Other Topics
Prerequisites:
- Benzene - Electrophilic aromatic substitution
- Alkyl Halides - Comparison with haloarenes
- Diazonium Salts - Sandmeyer reaction
Related Topics:
- Phenols - From haloarenes via SNAr
- Aromatic Amines - Synthesis planning
- Directing Effects - Electrophilic substitution
Applications:
- DDT and Pesticides - Environmental chemistry
- Industrial Solvents - Chemical industry
Teacher’s Summary
1. Haloarenes vs Haloalkanes (MOST IMPORTANT DISTINCTION)
- Haloarenes: X on aromatic ring, unreactive in nucleophilic substitution
- Haloalkanes: X on aliphatic chain, reactive
2. Why Unreactive:
- Resonance: C-X partial double bond
- sp² carbon: Higher s-character
- Phenyl cation: Extremely unstable
3. Preparation Methods:
- Direct halogenation: Benzene + X₂/Lewis acid
- Sandmeyer: Best method, from diazonium salt
- For F: Balz-Schiemann (diazonium fluoroborate)
4. Activated Nucleophilic Substitution (SNAr):
- Requires electron-withdrawing groups at o/p
- Addition-elimination mechanism
- Meisenheimer complex intermediate
5. Electrophilic Substitution:
- ortho/para directing (resonance donation)
- Deactivating (inductive withdrawal)
- Slower than benzene
6. Synthesis Strategy:
- For meta: Use NO₂ or other meta-director first
- For ortho/para: Use o/p-director first
- Plan backward from target!
“Haloarenes are unreactive in substitution but follow normal electrophilic substitution - understand WHY for JEE success!”
Next: Study polyhalogen compounds to learn about CHCl₃, CCl₄, DDT, and CFCs!