CBSE Class 12 · 2024 · Haloalkanes and HaloarenesShort Answer

(a) In AgCN the Ag–C bond is largely covalent, so the carbon lone pair is unavailable; the nitrogen attacks the alkyl halide instead:

(b) Allyl chloride ionises to the \textbf{allyl carbocation}, CH${}_2$=CH–CH${}_2^{+}$ ↔ ${}^{+}$CH${}_2$–CH=CH${}_2$ — resonance delocalisation makes this cation unusually stable, so the S${}_{\mathrm{N}}$1 slow step is fast.
(c) In haloarenes: (1) resonance gives C–X partial double-bond character; (2) the sp${}^2$ carbon binds X more tightly; (3) the π-electron cloud repels incoming nucleophiles; (4) the aryl cation needed for S${}_{\mathrm{N}}$1 is highly unstable. Together these make nucleophilic substitution extremely difficult.