🔹 1. Preparation of Halogenoalkanes
(a) Free-Radical Substitution of Alkanes
- Reaction type: Substitution
- Example: Ethane + Chlorine → Chloroethane + HCl
- Conditions: UV light (photochemical reaction)
- Mechanism:
- Initiation: Cl₂ → 2Cl• (via UV light)
- Propagation:
- Cl• + C₂H₆ → C₂H₅• + HCl
- C₂H₅• + Cl₂ → C₂H₅Cl + Cl•
- Termination: Combination of radicals
- Note: Side reactions lead to multiple halogenation products (e.g. CH₃CHCl₂)
(b) Electrophilic Addition of Alkenes
- Reagents:
- Halogen (Cl₂ or Br₂)
- Hydrogen halide (HCl, HBr)
- Conditions: Room temperature
- Example: Ethene + HBr → Bromoethane
- Mechanism:
- Double bond acts as nucleophile
- Forms carbocation intermediate (in HX addition)
- Markovnikov’s rule applies (H adds to C with more H atoms)
- Product: Halogenoalkane
(c) Substitution of Alcohols
| Reagents | Conditions | Reaction Type | Example |
|---|---|---|---|
| HX(g) | Heat with concentrated acid (e.g. HCl + ZnCl₂ for 1° alcohols) | Substitution | CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O |
| KCl + conc. H₂SO₄ / H₃PO₄ | Heat | Substitution | |
| PCl₃ | Heat | Substitution | 3CH₃CH₂OH + PCl₃ → 3CH₃CH₂Cl + H₃PO₃ |
| PCl₅ | Room temperature | Substitution | CH₃CH₂OH + PCl₅ → CH₃CH₂Cl + POCl₃ + HCl |
| SOCl₂ | Heat | Substitution | CH₃CH₂OH + SOCl₂ → CH₃CH₂Cl + SO₂ + HCl |
🔹 2. Classification of Halogenoalkanes
- Based on the carbon bonded to the halogen:
- Primary (1°): Halogen attached to a C bonded to 1 alkyl group
- Secondary (2°): Halogen attached to a C bonded to 2 alkyl groups
- Tertiary (3°): Halogen attached to a C bonded to 3 alkyl groups
🔹 3. Nucleophilic Substitution Reactions
(a) Reaction with NaOH(aq)
- Reagents: Aqueous NaOH
- Conditions: Heat under reflux
- Mechanism: Nucleophilic substitution (SN1 or SN2)
- Product: Alcohol
CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻
(b) Reaction with KCN
- Reagents: KCN in ethanol
- Conditions: Heat under reflux
- Product: Nitrile (C≡N)
CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻
(c) Reaction with NH₃
- Reagents: Excess ammonia in ethanol
- Conditions: Heated under pressure
- Product: Primary amine
CH₃CH₂Br + NH₃ → CH₃CH₂NH₂ + HBr
(Further substitution possible if NH₃ not in excess)
(d) Identification using AgNO₃(aq) in Ethanol
- Purpose: Test for halide
- Reagents: Aqueous AgNO₃ in ethanol
- Conditions: Warm
- Observation:
- Cl⁻ → white ppt (AgCl)
- Br⁻ → cream ppt (AgBr)
- I⁻ → yellow ppt (AgI)
🔹 4. Elimination Reactions
- Reagents: Ethanolic NaOH
- Conditions: Heat under reflux
- Product: Alkene
CH₃CH₂Br + OH⁻ (ethanol) → CH₂=CH₂ + H₂O + Br⁻ - Competes with substitution – conditions determine pathway
🔹 5. Mechanisms: SN1 vs SN2
SN2 (Bimolecular)
- Occurs in: Primary halogenoalkanes
- One-step mechanism
- Backside attack of nucleophile
- Inversion of configuration
- Rate ∝ [halogenoalkane][nucleophile]
SN1 (Unimolecular)
- Occurs in: Tertiary halogenoalkanes
- Two-step mechanism:
- Step 1: Slow formation of carbocation
- Step 2: Fast nucleophilic attack
- Racemization occurs
- Rate ∝ [halogenoalkane] only
🔹 6. Reactivity Trends in Halogenoalkanes
Factors affecting reactivity:
- C–X bond strength:
- C–F > C–Cl > C–Br > C–I
(Weaker bonds break more easily → more reactive)
- C–F > C–Cl > C–Br > C–I
- Bond polarity:
- C is δ⁺ → attracts nucleophile
- Steric hindrance:
- Affects SN2 (bulky tertiary halogenoalkanes hinder nucleophile attack)
Experimental Evidence:
- Hydrolysis with AgNO₃(aq):
- Tertiary reacts fastest (via SN1)
- Primary reacts slowest (via SN2)
✅ Summary Table:
| Reaction | Reagents | Conditions | Product |
|---|---|---|---|
| Substitution (OH⁻) | NaOH(aq) | Heat under reflux | Alcohol |
| Substitution (CN⁻) | KCN in ethanol | Heat | Nitrile |
| Substitution (NH₃) | NH₃ in ethanol | Heat, pressure | Amine |
| Elimination | NaOH in ethanol | Heat | Alkene |
| Identification | AgNO₃ in ethanol | Warm | Precipitate of AgX |
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