Lecture Notes: Haloalkanes and Haloarenes

Introduction

• Welcome and greetings
• Focus of the lecture: Haloalkanes and Haloarenes
• Structure and plan: Theory, then practice with previous year questions

Haloalkanes and Haloarenes

Definitions

• Haloalkanes: Organic compounds containing halogens and alkane groups (open chain structures)
• Haloarenes: Organic compounds containing halogens and aromatic (cyclic) structures
• Halogens (X) include F, Cl, Br, I

Methods of Preparation

Haloalkanes

From Alcohols

• Reaction: Alcohol + HX → Haloalkane + Water
• Mechanism: Protonation → Leaving group formation → Carbocation formation → Nucleophilic attack by X⁻
• Example:
  • Alcohol (ROH) reacts with HX
  • Forms R-OH₂⁺ → R⁺ (carbocation) → RX (haloalkane)
• Rearrangement: Possible hydride/methyl shifts for more stable carbocation

From Alkanes (Halogenation)

• Reaction: Alkane + X₂ (Cl₂ or Br₂) → Haloalkane
• Mechanism: Free radical substitution
• Selectivity: Br₂ is more selective and less reactive; Cl₂ is more reactive and less selective

From Alkenes

• Reaction: Alkene + X₂ in CCl₄ (non-polar solvent) or H₂O (polar solvent)
• Mechanism:
  • Non-polar solvent: Cyclic halonium ion formation → Nucleophilic attack (anti-addition)
  • Polar solvent: Similar process but with water as nucleophile

Hydrohalogenation of Alkenes

• Reaction: Alkene + HX → Haloalkane
• Mechanism: Carbocation formation → Rearrangement → Nucleophilic attack

Halogen Exchange (Finkelstein and Swarts Reaction)

• Swarts Reaction: R-X + AgF/Hg₂F₂ → R-F + AgX/Hg₂X₂
• Finkelstein Reaction: R-X + NaI (in acetone) → R-I + NaX

Methods of Preparation of Haloarenes

Electrophilic Substitution Reactions

• Example: Benzene + X₂ (in presence of AlCl₃) → Haloarene + HCl
• Mechanisms: Formation of positively charged halogen ion (X⁺)

From Aniline (Diazotization)

• Reaction: Aniline + NaNO₂/HCl (0-5°C) → Diazonium salt
• Substitution: CuX (Sandmeyer Reaction) for Cl, Br, I; HBF₄ for F
• Products: Ar-X (where X = Cl, Br, I, F)

Properties of Haloalkanes

Nucleophilic Substitution Reactions

• SN1 Mechanism: Two-step process; formation of carbocation intermediate; favored by polar protic solvents
• SN2 Mechanism: One-step process; simultaneous bond-breaking and bond-forming; favored by polar aprotic solvents
• Stereochemistry:
  • SN1: Racemization (near 50% inversion and 50% retention)
  • SN2: 100% inversion

Energy Profile Diagrams

• SN1: Two transition states due to two-step mechanism
• SN2: Single transition state

Factors Affecting Rate of Reaction

• SN1: Stability of carbocation (3° > 2° > 1°)
• SN2: Steric hindrance (1° > 2° > 3°)
• Leaving Group Ability: Down the group in halogens (I⁻ > Br⁻ > Cl⁻ > F⁻)
• Solvents: Polar protic for SN1; Polar aprotic for SN2

Special Reactions

• E2 Reactions: Elimination to form alkenes; anti-elimination mechanism
• Reaction with Metals: Wurtz Reaction (Alkyl halide + Na → Alkane); Formation of Grignard reagents (RMgX)

Properties of Haloarenes

Nucleophilic Substitution Reactions

• Conditions: High temp. and pressure; presence of electron-withdrawing groups (EWG) like NO₂
• Mechanism: Addition-Elimination (benzyne intermediate formation)

Electrophilic Substitution Reactions

• Example Reactions:
  • Halogenation: Ar-X + X₂/FeX₃ → Ar-X₂
  • Nitration: Ar-X + HNO₃/H₂SO₄ → Ar-X-NO₂
  • Sulfonation: Ar-X + H₂SO₄ → Ar-X-SO₃H
  • Friedel-Crafts Alkylation: Ar-X + RCl/AlCl₃ → Ar-X-R
  • Friedel-Crafts Acylation: Ar-X + RCOCl/AlCl₃ → Ar-X-COR

Summary

• Discussed methods of preparation and properties of Haloalkanes and Haloarenes
• Covered nucleophilic and electrophilic substitution reactions
• Special conditions for various reactions were highlighted