Account for the following:
(i) `pK_(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 feric 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.
Diazonium salts of aromatic amines are more stable than those of aliphatic amines.
(vii) Gabriel phthalimide synthesis is preferred for synthesising primary amines.

Explanation

(i) In aniline, the lone pair of electrons on N atom is delocalized over the benzene ring. As a result, electron density on the nitrogen decreases. On the other hand, in `CH_(3)NH_(2)`, +I effect of `CH_(3)` group increases the electron density on N atom. Therefore, aniline is less basic than methylamine and hence `pK_(b)` of aniline is higher than that of methylamine.
(ii) Ethylamine dissolves in water due to intermolecular hydrogen bonding as shown below :
`H-underset(C_(2)H_(5))underset(|)overset(H)overset(|delta-)N...... overset(delta+)H-underset(H)underset(|)overset(delta-)O...... overset(delta+)H-underset(C_(2)H_(5))underset(|)overset(H)overset(|delta-)N.....overset(delta+)H-underset(H)underset(|)O...`
However, because of large hydrophobic part (i.e., hydrocarbon part) of aniline, the extent of hydrogen bonding is less and therefore, aniline is insoluble in water.
(iii) Methylamine is more basic than water and therefore, accepts a proton from water forming `OH^(-)` ions.

These `OH^(-)` ions combine with `Fe^(3+)` ions to form brown ppt. of hydrated ferric oxide.
`FeCl_(3) to Fe^(3+) + 3Cl^(-)`
`2Fe^(3+) + 6OH^(-) to underset("(Brown ppt)")underset("Hydrated ferric oxide")(2Fe(OH)_(3)` or `Fe_(2)O_(3).3H_(2)O)`
(iv) Under strongly acidic conditions of nitration (in the presence of a mixture of conc. `HNO_(3) + H_(2)SO_(4)`), aniline gets protonated and is converted into anilinium ion having `-NH_(3)""^(+)`,group. This group is deactivating group and is m-directing. So, the nitration of aniline gives o-, p-nitroaniline (mainly p-product) while the nitration of anilinium ion gives m-nitroaniline.

Thus, nitration of aniline gives a substantial amount of m-nitroaniline due to protonation of aniline.
(v) Aniline being a Lewis base reacts with Lewis acid such as `AlCl_(3)` to form a salt.
`underset("Lewis base")(C_(6)H_(5)NH_(2)) + underset("Lewis acid")(AlCl_(3)) to underset("Salt")(C_(6)H_(5)NH_(2)^(+)AlCl_(3)""^(-)`
As a result, N of aniline acquires +ve charge and hence it acts as a strong deactivating group for electrophilic substitution reaction. Hence aniline does not undergo Friedel Crafts reaction.
(vi) The diazonium salts of aromatic amines are more stable than those of aliphatic amines because of dispersal of positive charge on the benzene ring due to resonance :

This type of resonance stability is not possible in alkyl diazonium salts.
(vii) Gabriel phthalimide reaction gives pure `1^(@)` amines without any impurity of `2^(@)` or `3^(@)` amines. Therefore, it is preferred for the synthesis of `1^(@)` amines.