Q. 4 C3.7( 7 Votes )

Arrange the following:

In increasing order of basic strength:

(a) Aniline, p-nitroaniline and p-toluidine

(b) C6H5NH2, C6H5NHCH3, C6H5CH2NH2

Answer :

(a) Electron-donating groups such as –CH3, -OCH3, -NH2, etc. increase the basicity and electron-withdrawing groups such as –NO2, -CN, -SO3H, -COOH, -X(halogen), etc. decrease the basicity of amines.


Explanation: Electron-donating groups releases electrons, stabilizes the conjugate acid (cation) and thus increases the basic strength.


Electron-withdrawing groups withdraws electrons, destabilizes the conjugate acid (cation) and thus decreases the basic strength.



In p-nitroaniline, NO2 group is present. As we know that NO2 group is an electron withdrawing group which decreases the basic strength of amine. In p-toluidine, CH3 group is present and as we know that CH3 group is an electron donating(releasing) group which increases the basic strength of amine.


Hence, p-toluidine is more basic than p-nitroaniline


Now in C6H5NH2 (aniline), due to delocalisation of lone pair of electrons of the N-atom over the benzene ring (decreases the electron density of N-atom) makes it less basic than p-toluidine but more basic than p-nitroaniline (NO2 group is present which decreases the density of aniline)


Combining all these facts, the relative basic strength of these three amines increases in the order:


p-nitroaniline< aniline < p-toluidine


(b) C6H5NH2, C6H5NHCH3, C6H5CH2NH2


In C6H5NH2 and C6H5NHCH3, the N-atom is directly attached to the aromatic ring. Hence, they both show resonance in which delocalisation of lone pair of electrons N-atom takes palce. As a result the electron density of N-atom decreases. Hence both are weaker bases. However in C6H5NHCH3, CH3 group(electron withdrawing) is present which increases the overall density of electrons.


Hence, C6H5NHCH3 is more basic than C6H5NH2


In C6H5CH2NH2, the N-atom is not directly attached to the aromatic ring. As a result, it does not show resonance. There is no effect on the electron density of lone pair of electrons of N-atom. Hence, it is more basic than C6H5NH2 and C6H5NHCH3


Combining all these facts, the relative basic strength of these three amines increases in the order:


C6H5NH2 < C6H5NHCH3 < C6H5CH2NH2


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