N,N,2,6 - Tetramethylaniline is more basic than N,N - dimethylaniline. Explain.

Explanation

Because of steric interaction involving two ortho - methyl groups and two methyl groups attached to nitrogen, the unshared electron - pair on N is not involed in resonance interaction with `pi` - electrons (steric inhibition of resonance) of ring. In fact, to avoid steric strain, - `NM_(2)` groups rotates about C - N bond axis and thereby loses coplanarity with the ring. As a result, nitrogen can easily donate its unshared electron - pair to a proton. On the other hand, no such steric inhibition occurs in N,N - diemthylaniline because the two ortho H-atoms are relatively much smaller in size. The unshared electron - pair on N - atom become involed in resonance interaction with the ring and therefore, is not fully available for taking up a proton. This explains why N,N,2,6 - tetramethylaniline is more basic than N,N - dimethylanline.