Q.(a) Aryl halides are less reactive in nucleophilic substitution reactions.

Aryl halides resist SN reactions mainly because of C–X resonance stabilisation and sp2-carbon bond strength; alkyl halides are commonly made from alcohols via HX/ZnCl2 or PCl3/PCl5/SOCl2; and among the given options, chlorobenzene (mono-halogenated) is not a 'poly'-halogen compound.

(a)(i) Why aryl halides are less reactive in nucleophilic substitution

1. Resonance: the halogen's lone pair conjugates with the aromatic π-system, giving the C–X bond partial double-bond character; this makes the bond shorter and stronger, so it resists cleavage by a nucleophile.
2. Hybridisation/bond strength: the carbon bearing the halogen is sp2 hybridised; sp2 carbon has greater s-character than sp3, holding the shared electron pair (and hence the halogen) more tightly, giving a stronger, harder-to-break C–X bond. Also, the electron-rich benzene ring repels an approaching nucleophile, and formation of the required carbocation/transition state (phenyl cation is very unstable) is difficult.

(a)(ii) Example of nucleophilic substitution in aryl halides

Chlorobenzene reacts with aqueous NaOH only under forcing conditions:

C6H5Cl + NaOH(aq) --623 K, 300 atm--> C6H5OH + NaCl (Dow's process for phenol manufacture)

(b) Preparation of alkyl halides

A common method is from alcohols: reaction with HCl in the presence of anhydrous ZnCl2 (Lucas reagent), or with PCl5, PCl3, or SOCl2 (the SOCl2 route, with pyridine, is preferred since by-products are gaseous and escape, giving pure product):

R–OH + HCl --ZnCl2--> R–Cl + H2O

R–OH + PCl5 → R–Cl + POCl3 + HCl

3R–OH + PCl3 → 3R–Cl + H3PO3

R–OH + SOCl2 --pyridine--> R–Cl + SO2 + HCl …