NCERT Class XII Chemistry
Chapter - Amines
Questions with Solutions
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Question : 18
Total: 22
Arrange the following in increasing order of their basic strength.
(a)C 2 H 5 N H 2 , C 6 H 5 N H 2 N H 3 , C 6 H 5 C H 2 N H 2 ( C 2 H 5 ) 2 N H
(b)C 2 H 5 N H 2 , ( C 2 H 5 ) 2 N H ( C 2 H 5 ) 3 N , C 6 H 5 N H 2
(c)C H 3 N H 2 , ( C H 3 ) 2 N H ( C H 3 ) 3 N , C 6 H 5 N H 2 C 6 H 5 C H 2 N H 2
(a)
(b)
(c)
Solution:
(a) The order of basicity is
C 6 H 5 N H 2 < N H 3 < C 6 H 5 C H 2 N H 2 < C 2 H 5 N H 2 < ( C 2 H 5 ) 2 N H
The basic nature of amine arises from their ability to donate the lone pair of electrons on N to electrophiles. The availability of this l.p. depends on two factors:
(i) Electron donating/withdrawing effect of the alkyl groups attached to the N atom.
(ii) Steric hindrance posed by alkyl groups around N.
The presence of alkyl groups on the N atom increases the electron density and makes the l.p. more available for electrophiles. This happens due to the +I effect of the alkyl groups. But, if the alkyl groups are too bulky or too many in number, they tend to sterically hinder the incoming proton and the basic strength decreases. These two factors working in opposing directions, tend to balance out each other in 2° amines, making them most basic and the basic strength follows the order:
(b)
<
<
<
(c)C 6 H 5 N H 2 < C 6 H 5 C H 2 N H 2 < ( C H 3 ) 3 N < C H 3 N H 2 < ( C H 3 ) 2 N H
The basic nature of amine arises from their ability to donate the lone pair of electrons on N to electrophiles. The availability of this l.p. depends on two factors:
(i) Electron donating/withdrawing effect of the alkyl groups attached to the N atom.
(ii) Steric hindrance posed by alkyl groups around N.
The presence of alkyl groups on the N atom increases the electron density and makes the l.p. more available for electrophiles. This happens due to the +I effect of the alkyl groups. But, if the alkyl groups are too bulky or too many in number, they tend to sterically hinder the incoming proton and the basic strength decreases. These two factors working in opposing directions, tend to balance out each other in 2° amines, making them most basic and the basic strength follows the order:
In aryl amine, the l.p. on N is involved in resonance and hence less available to proton.
(b)
(c)
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