CBSE Class 12 Chemistry: Haloalkanes & Haloarenes — Notes 2026

Preparation of Haloalkanes

• From alkanes: free radical halogenation (UV light); Cl₂ > Br₂ (reactivity), Br₂ > Cl₂ (selectivity)
• From alkenes: HX addition (Markovnikov's rule for HX; anti-Markovnikov with peroxide)
• From alcohols: ROH + HX; order of reactivity HI > HBr > HCl; Lucas test distinguishes alcohols

Types of Haloalkanes

• Primary (1°): halogen on carbon attached to one alkyl group; CH₃CH₂Cl
• Secondary (2°): halogen on carbon between two alkyl groups; (CH₃)₂CHCl
• Tertiary (3°): halogen on carbon with three alkyl groups; (CH₃)₃CCl

SN1 and SN2 Reactions

• SN2: bimolecular; nucleophile attacks backside while leaving group leaves; one step; primary > secondary > tertiary (steric hindrance)
• SN1: unimolecular; first step: carbocation formation; second step: nucleophile attacks; tertiary > secondary > primary (carbocation stability)
• Stereochemistry: SN2 → inversion (Walden inversion); SN1 → racemisation (both configurations formed)

Elimination Reactions

• E2: concerted elimination; strong base; anti-periplanar arrangement; primary alkyl halides prefer E2
• E1: two steps; carbocation forms first; then proton removed by base; tertiary alkyl halides
• Saytzeff's rule: more substituted alkene (stable) is major product in elimination

Haloarenes (Aryl Halides)

• Aryl halides: halogen directly on benzene ring; C−X bond has partial double bond character (resonance)
• C−X bond stronger than in alkyl halide; aryl halides less reactive to nucleophilic substitution
• Electrophilic substitution: −X is ortho-para director (activating through induction; deactivating through resonance)

Uses and Environmental Concerns

• CFCs (Freons): refrigerants, aerosol propellants; depletes ozone; phased out under Montreal Protocol
• DDT: pesticide; banned due to biomagnification; persists in environment; neurotoxin
• Chloroform: anaesthetic (obsolete); carbon tetrachloride: solvent; halothane: anaesthetic in surgery

CBSE Board Focus

• Haloalkanes: 5–7 marks; SN1 vs SN2 mechanism, Markovnikov's rule, elimination
• Draw mechanism for SN2: show backside attack, transition state, inversion product
• Distinguish: aryl vs alkyl halide reactivity; reason for low reactivity of aryl halides