Question

Account for the following:
(i) $p{K_b}$ 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.

Answer 1

Hint : This is a reasoning question one has to be familiar with all the terms involved let us know about the first term $p{K_b}$ it is the negative base ($10$) logarithm of the dissociation constant (basic) ( ${K_b}$ ) of a solution. It is used to determine the strength of a base or an alkaline solution. The equation to represent is as follows:
$p{K_b} = - lo{g_{10}}{K_b}$
The lower is the value of$p{K_b}$, the stronger is the base obtained.

Complete step by step solution :
Let us start with giving reasons for the first part of the question,
(i)The $p{K_b}$ of aniline is more than the methylamine because, In aniline the Nitrogen atom is directly attached to the benzene ring and therefore the lone pair on a Nitrogen atom is delocalized over the entire benzene ring, as benzene ring undergoes or exhibits resonance, due to this aniline cannot accept a proton, on the other hand in methylamine the electron density on nitrogen is greater than that of aniline as there is a presence of methyl group which has a tendency to donate its electron density via inductive effect (type of an interaction where a sigma bond transfers or takes up electron density to/from an adjacent atom) thus increasing the electron density at the nitrogen atom.
(ii) Ethylamine is soluble in water whereas aniline is not this is because ethylamine is capable of forming hydrogen bonds with water easily but in aniline due to the bulkiness of the carbons in the benzene ring it prevents the formation of effective hydrogen bonding and resulting being insoluble in water.
(iii) Methylamine in water reacts with ferric chloride because of the basic strength of $C{H_3}N{H_2}$. The Inductive effect of methyl group increases the electron density at nitrogen atom and makes it more basic than that of water and in water methylamine tends to produce ${[OH]^ - }$ ions resulting in which there is a formation of a precipitate hydrated ferric oxide.

Note : A student should not confuse between the two sub-classes of Inductive effect one is the $+ I$and the other is the $- I$ effect where the groups exhibiting $+ I$ effect transfers electron density to the adjacent atom whereas $- I$ effect exhibiting group takes up electron density from the adjacent atom.