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 the (N) atom is delocalised over the benzene ring. As a result , the electron density on nitrogen decreases . In contrast. In ` CH_3 NH_2 (+I)` effect of ` CH_3` increases the electron density on the N atom . Therefore, aniline is a weaker base than emthylamin and hence its ` pK_b` value is higher than that methylamine .
ii. Ethylamine dissoves in water due to intermoleuclar H-bonding as shpwn be,pw :

However , in aniline, due to the large hydroplobic part . i.e. , hydrocarbon part , the extent of H-bonding decreases considerably and hence aniline is insoluble in water. .
ii.
Being more baisc than water, methylamine accepts a proton from liberation ` overset (Θ) (OH)` ions .
.
The `overset (Θ) (OH)` ions combine with ` Fe^(3+)` ions pressent in ` H_2O` to from a brow prectipitate of hydrated ferric oxide .
` FeCl_3 rarr Fe^(3+) + 3 Cl^-`
`2Fe^(3)+6overset(Theta)OHrarr underset("Brown ppt")underset("Hydrated ferric oxide")(2Fe(OH)_(3)or Fe_(2)O_(3).3H_(3)O)`
iv. In acidic medium , aniline is protonated to give anilinium ion which is m-directiong.
v.Because aniline reacr4s with ` AlCl_3` to form salt .
. vi. The diazonium salts of aromatic amines are more stable than those of aliphatic amines due to disjpersal of the positive charges on the benzene ring as shown below :