Amines - Result Question 23
####25. What is the decreasing order of basicity of primary, secondary and tertiary ethylamines and $NH_3$ ?
[1994]
(a) $NH_3>C_2 H_5 NH_2>(C_2 H_5)_2 NH>$
$ (C_2 H_5)_3 N $
(b) $(C_2 H_5)_3 N>(C_2 H_5)_2 NH>$
$ C_2 H_5 NH_2>NH_3 $
(c) $(C_2 H_5)_2 NH>C_2 H_5 NH_2>$
$ (C_2 H_5)_3 N>NH_3 $
(d) $(C_2 H_5)_2 NH>(C_2 H_5)_3 N>$
$ C_2 H_5 NH_2>NH_3 . $
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Solution:
- (d) All aliphatic amines are stronger bases than $NH_3$ and among different ethylamines order of basictity is $2^{\circ}>3^{\circ}>1^{\circ}$. Thus, the correct order is (d) (in polar solvent):
$ \begin{aligned} & (C_2 H_5)_2 NH>(C_2 H_5)_3 N> \\ & \quad C_2 H_5 NH_2>NH_3 . \end{aligned} $
This anomalous behaviour of tertiary ethyl amine is due to steric factors i.e., crowding of alkyl groups cover nitrogen atom from all sides and thus makes the approach and bonding by a lewis acid relatively difficult which results the maximum steric strain in tertiary amines. The electrons are there but the path is blocked resulting the reduction in its basicity.
This comparison is true for amines in a protic solvent like water. In gaseous phase, the order of basicity is: tertiary amine $>$ secondary amine $>$ primary amine $>NH_3$.
