Haloalkanes and haloarenes
Class 12 · Updated 2026-10-03. Published for practice.
Preparation
From alcohols: ROH + HX or PCl5 / SOCl2. From alkenes: addition of HX (Markovnikov for unsymmetrical alkenes unless peroxide for HBr anti-Markovnikov).
Halogen exchange (Finkelstein), Sandmeyer and Gattermann for aryl halides from diazonium salts — know one-line conditions for each name reaction NCERT highlights.
Nucleophilic substitution
SN2: one step, backside attack, inversion of configuration, rate depends on [substrate][Nu]. Favoured by primary halide, good nucleophile, polar aprotic solvent.
SN1: two steps, carbocation intermediate, racemisation possible, rate depends on [substrate]. Tertiary halides, weak nucleophile, polar protic solvent favour SN1.
Aryl halides: C–X bond partial double character; nucleophilic substitution is hard unless activated (e.g. ortho/para nitro groups) — know contrast with alkyl halides.
Elimination and environment
E2 often competes with SN2 (strong base, heat). Saytzeff rule: more substituted alkene usually dominates in dehydrohalogenation.
Polyhalogen compounds: CHCl3, CCl4 uses and toxicity; iodoform test (CHI3 from methyl ketones or alcohols oxidisable to methyl ketones) is a practical identification link.
CFCs and ozone — environmental chemistry crossover from Class 11.