Among the following, the strongest base is: (IIT-JEE, 2000)
A
C6H5NH2
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B
p−NO2C6H4NH2
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C
m−NO2C6H4NH2
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D
C6H5CH2NH2
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Solution
The correct option is DC6H5CH2NH2 Basicity is a direct consequence of whether the lone pair of nitrogen electrons are available or not. So all aliphatic amines will be more basic than an aromatic amine. This is because the lone pair of electrons will be in plane with the benzene pi electrons involved in resonance. This makes them practically unavailable for boding.
(d) is an aliphatic amine; so it is stronger base than aromatic amine.
The second thing you need to know is the nature of the group on the benzene and its position. an electron withdrawing group or EWG would further decrease basisity as it will pull electrons towards it, away from the amine electrons. So the −NO2 group at o, m, or p positions decreases the basic character. The basicity order would be : (d) > (a) > (c) > (b).