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Question

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 ferric oxide.


Solution

(i) $$pK_b$$ of aniline is more than that of methylamine.
In aniline, the lone pair of electrons on $$N$$ atom is in resonance with benzene ring. Hence, it cannot be easily donated to an acid. This decreases its basicity. In methyl amine, the $$+I$$ effect of methyl group increases the electron density on $$N$$ atom so that the lone pair of electrons on $$N$$ atom can be easily donated to an acid. Hence, methylamine is more basic than aniline. Higher is the basicity, lower is the $$pKb$$ and vice versa. 

(ii) Ethylamine is soluble in water whereas aniline is not. With increase in the molecular weight, the solubility decreases. Aniline has higher molecular weight than ethylamine.

(iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide. Due to the $$+I$$ effect of $$-CH_3$$ group, methylamine is more basic than water. Therefore, in water, methylamine produces $$OH^-$$ ions by accepting $$H^+$$ ions from water. $$OH^-$$ ions react with ferric chloride to precipitate hydrated ferric oxide. 



Chemistry
NCERT
Standard XII

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