Chapter 10 — Haloalkanes and Haloarenes — is the gateway to organic reactions. This chapter introduces substitution and elimination reactions, which form the basis of organic chemistry. It carries 6-8 marks in Board exams. Master the SN1/SN2 mechanisms and named reactions like Wurtz, Fittig, and Sandmeyer.
Key Concepts
Classification
| Type | Structure | Example |
|---|---|---|
| Primary (1°) | −CH₂X | CH₃CH₂Cl (ethyl chloride) |
| Secondary (2°) | −CHX− | (CH₃)₂CHBr (isopropyl bromide) |
| Tertiary (3°) | −CX− | (CH₃)₃CCl (tert-butyl chloride) |
| Haloarene | X on benzene ring | C₆H₅Cl (chlorobenzene) |
Preparation Methods
R−OH + HX → R−X + H₂O (reactivity: HI > HBr > HCl)
R−OH + PCl₅ → R−Cl + POCl₃ + HCl
3R−OH + PCl₃ → 3R−Cl + H₃PO₃
R−OH + SOCl₂ → R−Cl + SO₂↑ + HCl↑ (best method — byproducts are gases!)
2. From Alkenes (Markovnikov’s addition):
CH₂=CH₂ + HBr → CH₃CH₂Br
3. Halogenation of Alkanes (free radical):
CH₄ + Cl₂ →(UV/hv) CH₃Cl + HCl
4. Sandmeyer Reaction (for ArX):
ArN₂⁺Cl⁻ + CuCl/HCl → ArCl + N₂
ArN₂⁺Cl⁻ + CuBr/HBr → ArBr + N₂
Nucleophilic Substitution Reactions
The C−X bond in haloalkanes is polar (C^δ⁺−X^δ⁻), making carbon susceptible to nucleophilic attack.
SN2 Mechanism (Substitution Nucleophilic Bimolecular)
One-step mechanism: nucleophile attacks from the back side (180° to leaving group)
Transition state: pentacoordinate carbon
Result: Walden inversion (configuration inverts — like an umbrella flipping)
Reactivity order: 1° > 2° > 3° (steric hindrance increases)
SN1 Mechanism (Substitution Nucleophilic Unimolecular)
Two steps: (1) Ionisation: R−X → R⁺ + X⁻ (slow, rate-determining)
(2) Nucleophilic attack: R⁺ + Nu⁻ → R−Nu (fast)
Carbocation intermediate: planar → nucleophile attacks from both sides
Result: Racemisation (mixture of d and l forms)
Reactivity order: 3° > 2° > 1° (more stable carbocation → faster ionisation)
Elimination Reactions
R−CH₂−CH₂−X + alc. KOH → R−CH=CH₂ + KX + H₂O
Saytzeff’s Rule: The more substituted alkene is the major product
CH₃CH₂CHBrCH₃ + alc. KOH → CH₃CH=CHCH₃ (major, more substituted) + CH₃CH₂CH=CH₂ (minor)
Reactions of Haloalkanes
| Nucleophile | Reagent | Product |
|---|---|---|
| OH⁻ | aq. KOH / NaOH | Alcohol (R−OH) |
| OR⁻ | NaOR (Williamson) | Ether (R−O−R’) |
| CN⁻ | KCN | Nitrile (R−CN) — carbon chain increases by 1! |
| NC⁻ | AgCN | Isocyanide (R−NC) |
| NH₃ | excess NH₃ | Amines (R−NH₂, R₂NH, R₃N, R₄N⁺) |
| NO₂⁻ | KNO₂ | Nitroalkane (R−NO₂) — via N |
| NO₂⁻ | AgNO₂ | Alkyl nitrite (R−O−N=O) — via O |
Important Named Reactions
(Used to increase carbon chain — makes symmetric alkanes)
Fittig Reaction: 2ArX + 2Na → Ar−Ar + 2NaX
Wurtz-Fittig: ArX + RX + 2Na → Ar−R + 2NaX
Finkelstein Reaction: R−Cl + NaI →(acetone) R−I + NaCl↓
(NaCl precipitates in acetone → drives reaction forward)
Swarts Reaction: R−Cl + AgF → R−F + AgCl↓
(Used to prepare fluoroalkanes)
Haloarenes — Why C−X Bond is Less Reactive
Chlorobenzene is much less reactive towards nucleophilic substitution than chloroalkanes because:
- Resonance: Lone pair of Cl delocalises into the ring → C−Cl gets partial double bond character → stronger, shorter bond
- sp² carbon: The carbon attached to Cl is sp² hybridised → more electronegative → holds electrons tighter
- Instability of phenyl cation: C₆H₅⁺ is very unstable → SN1 doesn’t occur easily
Reactions of Haloarenes
- Electrophilic substitution: Cl is ortho-para directing (due to +M effect) but deactivating (due to −I effect)
- Dow process: C₆H₅Cl + NaOH →(623 K, 300 atm) C₆H₅OH + NaCl
- Ullmann reaction: 2C₆H₅Cl + 2Cu →(Δ) C₆H₅−C₆H₅ (biphenyl)
Important Definitions
| Term | Definition |
|---|---|
| Nucleophilic Substitution | Reaction where a nucleophile replaces the leaving group |
| SN1 | Unimolecular nucleophilic substitution — rate depends on substrate only |
| SN2 | Bimolecular nucleophilic substitution — rate depends on both substrate and nucleophile |
| Walden Inversion | Inversion of configuration at the chiral centre during SN2 |
| Racemisation | Formation of equal amounts of d and l isomers (during SN1) |
| Elimination | Loss of HX from adjacent carbons to form a double bond |
Solved Examples — NCERT Based
Example 1: SN1 vs SN2
Q: Predict the mechanism (SN1 or SN2) for: (a) CH₃Br + OH⁻ (b) (CH₃)₃CBr + H₂O
Solution:
(a) CH₃Br is primary → SN2 (strong nucleophile OH⁻, no steric hindrance)
(b) (CH₃)₃CBr is tertiary → SN1 (stable 3° carbocation, weak nucleophile H₂O)
Example 2: Wurtz Reaction
Q: How would you prepare butane from ethyl bromide using Wurtz reaction?
Solution:
2CH₃CH₂Br + 2Na →(dry ether) CH₃CH₂−CH₂CH₃ + 2NaBr
Two ethyl groups couple to form butane (C₄H₁₀).
Example 3: Reactivity Order
Q: Arrange in decreasing order of SN2 reactivity: CH₃Cl, CH₃Br, CH₃I, CH₃F
Solution:
SN2 reactivity depends on the C−X bond strength (weaker bond = better leaving group):
C−I weakest → C−Br → C−Cl → C−F strongest
Order: CH₃I > CH₃Br > CH₃Cl > CH₃F
Example 4: Product Prediction
Q: What happens when chlorobenzene is treated with (a) Na in dry ether (b) NaOH at 623 K, 300 atm?
Solution:
(a) Fittig reaction: 2C₆H₅Cl + 2Na → C₆H₅−C₆H₅ (biphenyl) + 2NaCl
(b) Dow process: C₆H₅Cl + NaOH → C₆H₅OH (phenol) + NaCl
Important Questions for Board Exams
1 Mark Questions
- What is Walden inversion?
- Which is a better nucleophile: I⁻ or Cl⁻?
- Write the product of Finkelstein reaction of CH₃Cl with NaI.
- Why is chlorobenzene less reactive towards nucleophilic substitution?
2 Mark Questions
- Distinguish between SN1 and SN2 mechanisms (any 4 points).
- What is Saytzeff’s rule? Give an example.
- Why does KCN give cyanide but AgCN gives isocyanide?
- Write the Williamson ether synthesis reaction.
3 Mark Questions
- Explain the SN2 mechanism with an energy diagram. Why does it cause inversion?
- How do you convert: (a) ethanol to ethyl bromide (b) benzene to chlorobenzene (c) chlorobenzene to phenol?
- Give reasons: (a) Haloalkanes are more reactive than haloarenes (b) Allyl chloride is more reactive than n-propyl chloride towards SN1
5 Mark Questions
- Compare SN1 and SN2 reactions. Explain with mechanism, stereochemistry, and examples. What factors favour each?
- Discuss the preparation, properties and reactions of haloalkanes. Write any 4 nucleophilic substitution reactions with equations.
Quick Revision Points
- Best prep of R−Cl: R−OH + SOCl₂ (byproducts are gases)
- SN2: one step, back-attack, inversion, 1° best, strong nucleophile, aprotic solvent
- SN1: two steps, carbocation, racemisation, 3° best, weak nucleophile, protic solvent
- Elimination: alc. KOH → alkene (Saytzeff’s rule — more substituted product)
- Substitution: aq. KOH → alcohol
- KCN → R−CN; AgCN → R−NC (covalent silver salt → attacks through N)
- Wurtz: 2RX + 2Na → R−R; Fittig: 2ArX + 2Na → Ar−Ar
- Haloarenes: less reactive due to resonance (C−Cl partial double bond), ortho-para directing
- Finkelstein: R−Cl + NaI → R−I; Swarts: R−Cl + AgF → R−F
Chapter Navigation
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Related Chapters in Class 12 Chemistry
- Alcohols, Phenols and Ethers Class 12 Notes
- Aldehydes, Ketones and Carboxylic Acids Class 12 Notes
- Amines Class 12 Notes
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