Chapter 6: Haloalkanes & Haloarenes

Classification Based on Hybridization

• Alkyl Halides (R-X): Halogen bonded to sp³ hybridized carbon. (1°, 2°, or 3°).
• Allylic Halides: Halogen bonded to sp³ carbon next to a C=C double bond. Example: CH₂=CH-CH₂X.
• Benzylic Halides: Halogen bonded to sp³ carbon attached to an aromatic ring. Example: C₆H₅CH₂X.
• Vinylic Halides: Halogen bonded directly to sp² hybridized carbon of a C=C double bond. Example: CH₂=CH-X.
• Aryl Halides (Haloarenes): Halogen bonded directly to sp² carbon of an aromatic ring. Example: C₆H₅X.

Nature of C-X Bond

Since halogens are more electronegative than carbon, the C-X bond is highly polarized. Carbon acquires a partial positive charge (C^δ+) and halogen acquires a partial negative charge (X^δ-). Bond length increases from C-F to C-I, while bond enthalpy decreases.

From Alcohols (Best Methods)

• Using SOCl₂ (Darzen's Process): Preferred method because the byproducts (SO₂ and HCl) are escapable gases, leaving pure alkyl halide.
  R-OH + SOCl₂ → R-Cl + SO₂↑ + HCl↑
• Using Phosphorus Halides:
  3R-OH + PCl₃ → 3R-Cl + H₃PO₃
• Using Lucas Reagent (HCl + anhy. ZnCl₂): Used for primary and secondary alcohols (Groves' Process).

From Hydrocarbons

• Free Radical Halogenation: Cl₂/UV light yields a complex mixture of isomeric haloalkanes.
• Electrophilic Addition (Markovnikov's Rule): Addition of HX to unsymmetrical alkenes. The negative part of the addendum goes to the carbon with fewer hydrogen atoms.
• Peroxide Effect (Anti-Markovnikov / Kharasch Effect): Addition of HBr (ONLY HBr) in the presence of peroxide. Negative part (Br) goes to the carbon with MORE hydrogen atoms.

Halogen Exchange Methods (Crucial)

• Finkelstein Reaction (For Alkyl Iodides):
  R-X + NaI (in dry acetone) → R-I + NaX (X = Cl, Br)
  NaX precipitates in dry acetone, driving the reaction forward (Le Chatelier's Principle).
• Swarts Reaction (For Alkyl Fluorides): Heating R-Cl/R-Br with metallic fluorides like AgF, Hg₂F₂, CoF₃ or SbF₃.
  R-Br + AgF → R-F + AgBr

Electrophilic Substitution

Chlorination or Bromination of benzene/toluene in the presence of Lewis acid catalysts like Iron(III) chloride (FeCl₃) or AlCl₃ in dark.

C₆H₆ + Cl₂ (FeCl₃, dark) → C₆H₅Cl + HCl

From Diazonium Salts (Sandmeyer Reaction)

Primary aromatic amine (Aniline) treated with NaNO₂ + HCl at 273-278 K forms Benzene Diazonium Chloride. Treating this with Cu₂X₂ yields aryl halides.

Ar-N₂⁺Cl⁻ + Cu₂Cl₂ / HCl → Ar-Cl + N₂

Note: For Iodoarene, simply warm the diazonium salt with KI solution (No Copper catalyst required).

SN2 Mechanism (Bimolecular Nucleophilic Substitution)

• Kinetics: Follows 2nd order kinetics. Rate = k[R-X][Nu⁻].
• Mechanism: Single-step process. Nucleophile attacks from the back, forming a transition state where C is partially bonded to both Nucleophile and Halogen.
• Stereochemistry: Leads to complete Inversion of Configuration (Walden Inversion), like an umbrella turning inside out.
• Reactivity Order: Primary (1°) > Secondary (2°) > Tertiary (3°). Steric hindrance plays the main role.

SN1 Mechanism (Unimolecular Nucleophilic Substitution)

• Kinetics: Follows 1st order kinetics. Rate = k[R-X].
• Mechanism: Two-step process. Step 1 (Slow/Rate-determining): Cleavage of C-X bond to form a Carbocation. Step 2 (Fast): Attack of Nucleophile.
• Stereochemistry: Leads to Racemization (mixture of retention and inversion) because the carbocation is planar, allowing attack from both sides.
• Reactivity Order: Tertiary (3°) > Secondary (2°) > Primary (1°). Stability of Carbocation plays the main role (Hyperconjugation). Allylic and benzylic halides are highly reactive towards SN1.

Elimination Reactions (Beta-Elimination)

When heated with alcoholic KOH, haloalkanes lose HX to form alkenes (Dehydrohalogenation).

Zaitsev (Saytzeff) Rule: In case of unsymmetrical alkyl halides, the preferred alkene is the one which is more highly substituted (has more alkyl groups attached to the double-bonded carbons).

Why are Haloarenes less reactive towards Nucleophilic Substitution?

• Resonance Effect: C-Cl bond acquires partial double bond character.
• Hybridization: C atom is sp² hybridized (more electronegative), holding electron pair tighter.
• Instability of phenyl cation.
• Dow's Process: Chlorobenzene can be converted to Phenol only at extreme conditions (623 K, 300 atm) using NaOH. Presence of NO₂ group at ortho/para positions increases reactivity.

Reaction with Metals

• Wurtz Reaction: 2R-X + 2Na (dry ether) → R-R (Alkane)
• Fittig Reaction: 2Ar-X + 2Na (dry ether) → Ar-Ar (Diaryls/Biphenyl)
• Wurtz-Fittig Reaction: Ar-X + R-X + 2Na (dry ether) → Ar-R (Alkylarene)
• Grignard Reagent: R-X + Mg (dry ether) → R-Mg-X. (Highly reactive, must be prepared in anhydrous conditions to avoid reacting with moisture to form alkanes).

Polyhalogen Compounds & Environmental Impact

• Chloroform (CHCl₃): Oxidizes slowly in air to highly poisonous gas Phosgene (COCl₂). Stored in dark bottles completely filled.
• Freons (CFCs): Excellent refrigerants but deplete the ozone layer via free radical chain reactions in the stratosphere.
• DDT: Highly effective insecticide but non-biodegradable. Causes biomagnification and toxicity in the food chain.