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Question
Arrange the following.
In decreasing order of the pKb values C2H5NH2, C6H5NHCH3, (C2H)2NH and CH3NH2.
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Solution
Due to the delocalization of the lone pair of electrons of the N-atom over the benzene ring, C6H5NHCH3 is far less basic than C2H5NH2, (C6H5)2NH and CH3NH2. Among C2H5NH2 and (C2H5)NH, (C2H5)2NH is more basic than C2H5NH2 due to the greater +I effect of the two C2H5 groups and stabilization of its conjugate acid by H-bonding.
Compare to Ethyl and methyl group, C2H5 – group has a more +I effect than CH3 – group. Therefore methylamine is weak base than ethylamine. Combining all these facts the relative basic strength of these four amines decreases in order.
(C2H5)2NH > C2H5NH2 > CH3NH2 > C6H5NHCH3
Since a stronger base has a lower pKb value, therefore pKb values decrease in the reverse order.
C6H5NHCH3 > CH3NH2 > C2H5NH2 > (C2H5)2NH
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