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JEE Main levelSingle correct optionDifficulty 2 of 3nc117

The pKb of N,N-dimethylaniline is 8.85 and that of N,N,2-trimethylaniline is 8.14 (both in water). The higher basicity of the second compound is best explained by

  1. Athe ortho methyl group increases the +I effect on the ring, which is transmitted to nitrogen by resonance
  2. Bsteric inhibition of resonance: the ortho methyl twists the group out of the ring plane, so the lone pair is less delocalised
  3. Cthe ortho effect, which always increases the basicity of anilines
  4. Dhydrogen bonding between the ortho methyl group and the nitrogen lone pair

Answer: B

The lower pKb means the stronger base. In N,N,2-trimethylaniline the bulky ortho methyl forces the dimethylamino group out of the plane of the ring, so the nitrogen lone pair overlaps less with the pi system. The lone pair stays more available for protonation, which is steric inhibition of resonance.
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