Aliphatic Compounds Containing Nitrogen - Result Question 6
####6. In the following compounds, the decreasing order of basic strength will be
(2019 Main, 8 April I)
(a) $C _2 H _5 NH _2>NH _3>\left(C _2 H _5\right) _2 NH$
(b) $\left(C _2 H _5\right) _2 NH>NH _3>C _2 H _5 NH _2$
(c) $\left(C _2 H _5\right) _2 NH>C _2 H _5 NH _2>NH _3$
(d) $NH _3>C _2 H _5 NH _2>\left(C _2 H _5\right) _2 NH$
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Answer:
Correct Answer: 6. (c)
Solution:
- Basic strength can be compared by the reaction of an alkanamine and ammonia with proton.
Basicity of an amine in aqueous solution depends upon the solubility of ammonium cation formed by accepting proton from water. The stability of ammonium cation depends upon the following factors.
(a) $+I$ effect
(b) Steric effect
(c) Solvation effect
In first case, on increasing the size of alkyl group $+I$ effect increases and the positive charge of ammonium cation gets dispersed more easily, Therefore, order of basicity is $2^{\circ}$ amine > $1^{\circ}$ amine $>$ ammonia. In second case, substituted ammonium cation is also stabilised by solvation with water molecules. Greater the size of ion, lesser will be the solvation and less stabilised is the ion.
Considering both steric and solvation effect, it can be concluded that greater the number of $H$-atoms on the $N$-atom, greater will be the H-bonding and hence more stable is the ammonium cation. But in case of $-C _2 H _5$ group, $+I$ effect predominates over $H$-bonding. Therefore, order is $2^{\circ}$ amine $>1^{\circ}$ amine $>$ ammonia i.e. $\left(C _2 H _5\right) _2 NH>C _2 H _5 NH _2>NH _3$.
