Amines Ques 16
16. 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 $
Show Answer
Solution:
All aliphatic amines are stronger bases than $NH_3$ and among different ethylamines order of basicity is $3^{\circ}>1^{\circ}>2^{\circ}$. Thus, the correct order is (d) (in polar solvent)
$ \begin{alignedat} & (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 around the nitrogen atom from all sides and thus makes the approach and bonding by a lewis acid relatively difficult which results in the maximum steric strain in tertiary amines. The electrons are there but the path is blocked resulting in 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$_3$
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