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Haloalkanes & Haloarenes | Class 12 Organic Chemistry | SM-Educate

Haloalkanes & Haloarenes | Class 12 Organic Chemistry | SM-Educate
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CS-991 · Module O-XII1

Haloalkanes & Haloarenes

Class XII · Organic Chemistry
🎥 Video lesson coming soon

⚖️ Haloalkanes vs Haloarenes

Haloalkanes (R-X): halogen attached to an sp³ carbon of an alkyl chain. The C-X bond is a pure single bond, so haloalkanes react readily by nucleophilic substitution.

Haloarenes (Ar-X): halogen attached directly to an sp² carbon of an aromatic ring. The C-X bond gains partial double-bond character from resonance with the ring, making it shorter and stronger — so haloarenes are far less reactive towards nucleophilic substitution than haloalkanes.

🔧 Preparation

  • From alcohols: R-OH + HX / SOCl₂ / PCl₃ → R-X
  • From alkenes: addition of HX or X₂ across the double bond
  • Wurtz Reaction: 2 R-X + 2 Na → R-R + 2 NaX (used to make symmetrical alkanes by coupling two identical alkyl halides)

⚖️ Recap: SN1 vs SN2 (from Class XI)

Haloalkanes undergo nucleophilic substitution — tertiary substrates favour SN1 (via a carbocation), while primary substrates favour SN2 (single-step, backside attack, inversion of configuration).

🔄 Sandmeyer Reaction

Converts an aromatic diazonium salt (Ar-N₂⁺) into an aryl halide or nitrile, using a copper(I) salt as catalyst:

  • Ar-N₂⁺ + CuCl → Ar-Cl + N₂
  • Ar-N₂⁺ + CuBr → Ar-Br + N₂
  • Ar-N₂⁺ + CuCN → Ar-CN + N₂

This is important because direct halogenation of benzene rings is difficult to control — the Sandmeyer reaction gives a reliable, specific way to introduce Cl, Br, or CN onto a ring at a chosen position.

📝 Practice MCQs

Q1: Haloarenes are less reactive towards nucleophilic substitution than haloalkanes because:
a) They have a weaker C-X bond b) The C-X bond has partial double-bond character from resonance c) They lack a halogen d) They are not aromatic
Show Answer
✅ b) The C-X bond has partial double-bond character from resonance
Q2: The Wurtz reaction is used to prepare:
a) Aromatic compounds b) Symmetrical alkanes c) Alcohols d) Carboxylic acids
Show Answer
✅ b) Symmetrical alkanes
Q3: The Sandmeyer reaction converts a diazonium salt into an aryl halide using:
a) A copper(I) salt b) Sodium metal c) Concentrated H₂SO₄ d) A Grignard reagent
Show Answer
✅ a) A copper(I) salt
Q4: Tertiary haloalkanes typically react via:
a) SN2 b) SN1 c) No substitution possible d) Only elimination
Show Answer
✅ b) SN1
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