The dipole moment of N,N-dimethyl-4-nitroaniline (a) is 6.87 D, while that of its 2,3,5,6-tetramethyl derivative (b) is only 4.11 D.
(a) (b)
What best explains the lower dipole moment of (b)?

Question

The dipole moment of N,N-dimethyl-4-nitroaniline (a) is 6.87 D, while that of its 2,3,5,6-tetramethyl derivative (b) is only 4.11 D. (a) (b) What best explains the lower dipole moment of (b)?

TestHub · Accepted Answer

A large dipole in (a) comes from conjugation between the NMe₂ donor and the NO₂ acceptor through the ring. In (b) the ortho methyls twist the NMe₂ group out of the ring plane (steric inhibition of resonance), the lone pair can no longer delocalise toward the nitro group, and the dipole moment falls from 6.87 D to 4.11 D.

Chemistry - GOC Question with Solution | TestHub

Options:

Doubts & Discussion