Q.(a) An ambident nucleophile is

Nitrite ion is the ambident nucleophile; alkyl chlorides are made from alcohols via ZnCl2/HCl or SOCl2; aryl halides resist nucleophilic substitution because of C-X double-bond character from resonance.

(a) Ambident nucleophile

An ambident nucleophile has two different sites through which it can attack an electrophile, giving two possible products. Among ammonia, ammonium ion, chloride ion and nitrite ion, only the nitrite ion, NO2- (iv), is ambident - it can bond through the N atom (giving a nitro compound, R-NO2) or through an O atom (giving a nitrite ester, R-O-N=O).

(b)(i) Preparation of an alkyl chloride

A simple and general method is to react the corresponding alcohol with concentrated HCl in the presence of anhydrous ZnCl2 (Lucas reagent), which activates the -OH group as a good leaving group:

R-OH + HCl --(anhyd. ZnCl2)--> R-Cl + H2O

A cleaner, more widely used method (Darzens process) uses thionyl chloride, which gives the alkyl chloride along with two gaseous by-products that escape, leaving a pure product:

R-OH + SOCl2 -> R-Cl + SO2(g) + HCl(g)

(b)(ii) Why aryl halides resist nucleophilic substitution

1. Resonance: the lone pair on the halogen delocalises into the benzene ring, giving the C-X bond partial double-bond character. This makes the C-X bond shorter and stronger than in an alkyl halide, so it is much harder to break. …