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JEE Advanced levelSingle correct optionDifficulty 3 of 3g1101

Which of the following compounds has the larger dipole moment, and why? (P) 4-nitro-N,N-dimethylaniline, (Q) 3,5-dimethyl-4-nitro-N,N-dimethylaniline.

  1. ABoth are equal, since the substituents are the same
  2. BQ, because the twisted nitro group conjugates better with the ring
  3. CP, because the coplanar and groups give a strong push-pull conjugation, which Q loses as the nitro group is twisted
  4. DQ, because the two extra methyl groups add to the dipole by their +I effect

Answer: C

In P, the lone pair of the dimethylamino group is pushed through the ring to the nitro group, giving a large charge separation and a large dipole moment. In Q the two methyl groups flank the nitro group, which twists out of the plane (steric inhibition of resonance), so the -M effect is lost and the dipole moment falls.
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