Question

Arrange the following.

In decreasing order of the pK_b values C₂H₅NH₂, C₆H₅NHCH₃, (C₂H)₂NH and CH₃NH₂.

Answer 1

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 pK_b value, therefore pK_b values decrease in the reverse order.

C₆H₅NHCH₃ > CH₃NH₂ > C₂H₅NH₂ > (C₂H₅)₂NH