91影视

Greater the pKb value of the molecule or compound, lower will be basicity of that compound. The pKb value of 鈥淎鈥� is typical for an aniline, around 9. The pair of electrons from nitrogen is delocalised around the ring, stabilizing the molecule. The pKb value of 鈥淐鈥� is also typical of tertiary aliphatic amine, between 3 and 4. In 鈥淏鈥�, because of the rigidity of the alkyl groups, the pair of electrons in the orbital on nitrogen are perpendicular to the aromatic pi-system and there is no electron overlap between nitrogen and benzene ring, there is steric inhibition of resonance and benzene ring is electron-withdrawing by induction and it destabilizes the protonated form of 鈥淏鈥� making it less basic.

In 鈥淒鈥�, the two methyl groups at ortho positions prevents the two rings from being planar, effectively cutting off any electronic connection between the two rings. The nitrogen鈥檚 electrons in 鈥淒鈥� can still overlap with the first ring but they would not be delocalised over the second ring unlike in 鈥淓鈥�. Thus, 鈥淒鈥� would bbe as strong as a typical aniline and 鈥淓鈥� would be hardly basic at all.

There is steric inhibition of resonance involved in the given compound as presence of ortho methoxy groups forces the amines out of plane thereby, inhibiting the resonance with the ring. Hence, nitrogen atoms becomes free to donate the lone pair of electrons and on protonation, six-membered ring forms which increases the stability of conjugate acid, thereby largely increasing the basicity of the compound.