CBSE Class 12 · 2008 · All India · Haloalkanes and HaloarenesShort Answer

(i) In haloarenes the halogen lone pair is delocalised into the ring, giving the C–X bond partial double-bond character; the carbon is also sp${}^2$ (more electronegative, shorter/stronger bond), and the electron-rich ring repels nucleophiles. Haloalkanes have a simple polar C(sp${}^3$)–X bond that nucleophiles attack easily — hence they are far more reactive.
(ii) 3-Chloropentane ionises to the \textbf{secondary} carbocation CH${}_3$CH${}_2$\overset{+}{C}HCH${}_2$CH${}_3$, more stable than the primary cation from 1-chloropentane — so \textbf{3-chloropentane is faster in S${}_{\mathrm{N}}$1}.