Account for the following
(i) \(\mathrm{pK}_{\mathrm{b}}\) of aniline is more than that of methylamine
(ii) Ethylamine is soluble in water whereas aniline is not.
(iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide.
(iv) Although amino group is \(o\) and \(p\)-directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of \(m\) nitroaniline.
(v) Aniline does not undergo Friedel-Crafts reaction.
(vi) Diazonium salts of aromatic amines are more stable than those of aliphatic amines.
(vii) Gabriel phthalimide synthesis is preferred for synthesising primary amines.

Options:

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Solution:
(i) In aniline, the lone pair of electrons on the \(\mathrm{N}\)-atom is delocalised over the benzene ring. As a result, electron density on the nitrogen atom decreases. Whereas in \(\mathrm{CH}_3 \mathrm{NH}_2,+\) I-effect of \(-\mathrm{CH}_3\) group increases the electron density on the \(\mathrm{N}\)-atom. Therefore, aniline is a weaker base than methylamine and hence its \(\mathrm{pK}_{\mathrm{b}}\) value is higher than that of methylamine.
(ii) Ethylamine dissolves in water due to intermolecular H-bonding. However, in case of aniline, due to the large hydrophobic part, i.e., hydrocarbon part, the extent of \(\mathrm{H}\)-bonding is very less therefore aniline is insoluble in water.
(iii) Methylamine being more basic than water, accepts a proton from water liberating \(\mathrm{OH}^{-}\)ions.
These \(\mathrm{OH}^{-}\)ions combine with \(\mathrm{Fe}^{3+}\) ions present in \(\mathrm{H}_2 \mathrm{O}\) to form brown precipitate of hydrated ferric oxide.
\(\mathrm{FeCl}_3 \longrightarrow \mathrm{Fe}^{3+}+3 \mathrm{Cl}^{-}\)
\(2 \mathrm{Fe}^{3+}+6 \mathrm{OH}^{-} \longrightarrow \underset{\text{(Brown ppt.)}}{2 \mathrm{Fe}(\mathrm{OH})_3 \text{ or } \mathrm{Fe}_2 \mathrm{O}_3 .3 \mathrm{H}_2 \mathrm{O}}\)
(iv) Nitration is usually carried out with a mixture of conc \(\mathrm{HNO}_3+\) conc \(\mathrm{H}_2 \mathrm{SO}_4\). In presence of these acids, most of aniline gets protonated to form anilinium ion. Therefore, in presence of acids, the reaction mixture consist of aniline and anilinium ion. Now, \(-\mathrm{NH}_2\) group in aniline is activating and \(o, p\)-directing while the \(-\stackrel{+}{\mathrm{NH}_3}\) group in anilinium ion is deactivating and \(m\)-directing. Nitration of aniline (due to steric hindrance at \(o\)-position) mainly gives \(p\)-nitroaniline, the nitration of anilinium ion gives \(m\)-nitroaniline. In actual practice, approx a 1:1 mixture of \(p\)-nitroaniline and \(m\)-nitroaniline is obtained. Thus, nitration of aniline gives a substantial amount of \(m\)-nitroaniline due to protonation of the amino group.
(v) Aniline being a Lewis base reacts with Lewis acid \(\mathrm{AlCl}_3\) to form a salt.
\(\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2+\mathrm{AlCl}_3 \longrightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2{ }^{+} \mathrm{AlCl}_3{ }^{-}\)
As a result, \(\mathrm{N}\) of aniline acquires positive charge and hence it act as a strong deactivating group for electrophilic substitution reactions. Consequently, aniline does not undergo Fridel Crafts reaction.
(vi) The diazonium salts of aromatic amines are more stable than those of aliphatic amines due to dispersal of the positive charge on benzene ring as a result of resonance.
(vii) Gabriel phthalimide reaction gives pure primary amines without any contamination of secondary and tertiary amines. Therefore, it is preferred for synthesising primaryamines.

Topic: Chemistry, Amines