Learning Objectives
- Understand classification and nomenclature of haloalkanes and haloarenes
- Learn SN1 and SN2 mechanisms and their stereochemistry
- Study elimination reactions (E1 and E2)
- Understand reactivity differences between haloalkanes and haloarenes
- Learn about polyhalogen compounds (DDT, chloroform, freons)
Key Concepts
Classification
Haloalkanes: Halogen attached to sp³ carbon (R-X). Primary (1°), Secondary (2°), Tertiary (3°) based on carbon type.
Haloarenes: Halogen directly attached to aromatic ring (Ar-X). Less reactive than haloalkanes due to resonance.
Preparation
From alcohols: ROH + HX → RX + H₂O (reactivity: HI > HBr > HCl). ROH + PCl₅ → RCl + POCl₃ + HCl. ROH + SOCl₂ → RCl + SO₂ + HCl (best method, gaseous byproducts).
From alkenes: Addition of HX (Markovnikov's). Electrophilic addition of X₂.
Halogenation of alkanes: Free radical substitution with Cl₂/Br₂ in UV light.
Sandmeyer reaction: ArN₂⁺ + CuX → ArX + N₂ (for aryl halides from diazonium salts).
Nucleophilic Substitution Reactions
SN2 (Substitution Nucleophilic Bimolecular):
- One step: nucleophile attacks as leaving group departs (backside attack).
- Rate = k[RX][Nu⁻]. Bimolecular.
- Inversion of configuration (Walden inversion).
- Favoured by: 1° substrate (less steric hindrance), strong nucleophile, polar aprotic solvent.
SN1 (Substitution Nucleophilic Unimolecular):
- Two steps: (1) formation of carbocation, (2) nucleophile attack.
- Rate = k[RX]. Unimolecular.
- Racemisation (mixture of retention and inversion).
- Favoured by: 3° substrate (stable carbocation), weak nucleophile, polar protic solvent.
Elimination Reactions
Dehydrohalogenation: RX + alc. KOH → alkene + KX + H₂O.
Saytzeff's rule: More substituted (more stable) alkene is the major product.
E2: concerted, favoured by strong base, anti-periplanar geometry.
E1: stepwise via carbocation, favoured by weak base, 3° substrate.
Reactivity of Haloarenes
Haloarenes are less reactive toward nucleophilic substitution because:
- Resonance stabilisation (lone pair of halogen delocalised into ring).
- C-X bond has partial double bond character (shorter, stronger).
- sp² carbon of ring (more electronegative than sp³).
Reactions where aryl halides are reactive: Electrophilic aromatic substitution (halogen is o/p directing, deactivating). Ullmann reaction (ArX + Cu → Ar-Ar). Fittig reaction (ArX + 2Na + XAr → Ar-Ar). Wurtz-Fittig (ArX + 2Na + XR → Ar-R).
Polyhalogen Compounds
CHCl₃ (Chloroform): Anaesthetic, solvent. Oxidised by air to phosgene (COCl₂, toxic): 2CHCl₃ + O₂ → 2COCl₂ + 2HCl. Store in dark bottles with ethanol.
CCl₄ (Carbon tetrachloride): Fire extinguisher (Pyrene), solvent, refrigerant.
DDT: Insecticide, non-biodegradable, bioaccumulates.
Freons (CFCs): Refrigerants, propellants. Cause ozone depletion.
Summary
Haloalkanes undergo nucleophilic substitution (SN1/SN2) and elimination reactions. SN2 gives inversion, SN1 gives racemisation. Haloarenes are less reactive due to resonance but undergo electrophilic aromatic substitution. Reactivity order: C-I > C-Br > C-Cl > C-F (bond strength). Polyhalogen compounds have important applications but environmental concerns.
Important Terms
- SN1: Unimolecular substitution via carbocation, racemisation
- SN2: Bimolecular substitution, backside attack, inversion
- Walden Inversion: Inversion of configuration in SN2
- Saytzeff's Rule: More substituted alkene is major product
- Sandmeyer Reaction: Aryl halide from diazonium salt using CuX
Quick Revision
- SN2: 1° > 2° > 3°; SN1: 3° > 2° > 1° (carbocation stability)
- SN2: inversion; SN1: racemisation
- Reactivity: R-I > R-Br > R-Cl > R-F (C-X bond strength)
- Haloarenes: less reactive (resonance), o/p directing in EAS
- Best method for R-Cl from R-OH: SOCl₂ (Darzen's reaction)
- CHCl₃ + O₂ → COCl₂ (phosgene, toxic)