Question

The correct order of ${K_b}$ value of following is:
${({C_2}{H_5})_2}\mathop N\limits^{..} H,N{H_3},\mathop N\limits^{..} {({C_2}{H_5})_3}$
A. $1 > 2 > 3$
B. $1 > 3 > 2$
C. $3 > 2 > 1$
D. $3 > 1 > 2$

Answer 1

Hint: The basicity of a substance or compound is defined as the tendency of the base to lose electrons or to donate a lone pair of electrons to a Lewis acid. The various electronic effects such as inductive effect, mesomeric effect, hyperconjugation, etc. play an important role in deciding the basic character of a compound.

Complete step by step answer: In amines and substituted amines, the lone pair of electrons over the nitrogen atom are available for donation. There are some additional effects which can provide a gentle push to the lone pair of electrons for the easy donation. The electron withdrawing groups decrease the electron density from the compound and the electron donating groups increase the electron density over the compound. Thus, the electron donating tendency of the nitrogen atom is dependent directly on the availability of the electron donating groups attached directly to it.
${K_b} \propto \dfrac{{ + I, + M}}{{ - I, - M}}$
But, there is a variation in the gaseous and aqueous condition of substituted amine and normal amine.
The order of basic character in gaseous state is:
$Tertiary({3^o}) > Secondary({2^o}) >Primary({1^o})$
Thus, according to the question, the basic order in gaseous state will be:$\mathop N\limits^{..} {({C_2}{H_5})_3} > {({C_2}{H_5})_2}\mathop N\limits^{..} H > N{H_3}$
But, in the case of aqueous solution of amines, the basic order of the compounds will be: $Secondary({2^o}) > Tertiary({3^o}) > Primary({1^o})$
Thus, according to the question, the basic order is:
 $NH{({C_2}{H_5})_2} > N{({C_2}{H_5})_3} > N{H_3}$
The change in the order of basic strength in the aqueous state is because of the steric hindrance between the substituted groups. Due to this steric hindrance produced by the bulkier groups, it is difficult for the nitrogen atom to donate electrons easily. We will consider the basic order in the gaseous state itself because of the absence of order of aqueous phase in the provided options.
Thus, the correct option is C. $3 > 2 > 1$

Note: There is a difference between the basic order of substituted amines in aqueous state when the substituted groups are methyl and ethyl respectively.
For ethyl substituted amine, the basic order in aqueous state is: $NH{({C_2}{H_5})_2} > N{({C_2}{H_5})_3} > N{H_3}$. For methyl substituted amine, the basic order in aqueous state is: $NH{(C{H_3})_2} > N{H_3} > N{(C{H_3})_3}$