The dipole moment of 4-(dimethylamino)benzonitrile (a) is 6.15 D, while that of its 2,3,5,6-tetramethyl derivative (b) is only 3.85 D.
(a) (b)
What best explains the lower dipole moment of (b)?

Question

The dipole moment of 4-(dimethylamino)benzonitrile (a) is 6.15 D, while that of its 2,3,5,6-tetramethyl derivative (b) is only 3.85 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 NM e 2 ​ donor and the C ≡ N acceptor through the ring. In (b) the ortho methyls twist the NM e 2 ​ group out of the ring plane (steric inhibition of resonance), the lone pair can no longer delocalise toward the cyano group, and the dipole moment falls from 6.15 D to 3.85 D.

Chemistry - GOC Question with Solution | TestHub

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