Appeared in: 2022 (3 sets)
Give reasons :
(i) Ammonolysis of alkyl halides is not a good method to prepare pure primary amines.
(ii) Aniline does not give Friedel-Crafts reaction.
(iii) Although $- ext{NH}_2$ group is o/p directing in electrophilic substitution reactions, yet aniline on nitration gives good yield of m-nitroaniline.
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Official Evaluation Steps & EL10 Insight
Part (i): Ammonolysis of Alkyl Halides and Pure Primary Amines
Step 1 (Governing Principle & Reaction Equation): [0.5 Mark]
Ammonolysis of alkyl halides involves nucleophilic substitution where the nucleophile ($\text{NH}_3$) attacks the alkyl halide ($\text{R-X}$). The primary amine formed ($\text{R-NH}_2$) still contains a lone pair of electrons on nitrogen, making it a strong nucleophile that further reacts with remaining alkyl halide molecules in a stepwise manner:
$$\text{R-X} \xrightarrow[\text{-\text{HX}}]{\text{NH}_3} \underset{\text{(1° amine)}}{\text{R-NH}_2} \xrightarrow[\text{-\text{HX}}]{\text{R-X}} \underset{\text{(2° amine)}}{\text{R}_2\text{NH}} \xrightarrow[\text{-\text{HX}}]{\text{R-X}}