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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3g2219

Which of the following statements about amine basicity are correct? (The conjugate-acid pKa values in water are: 10.6, 9.25, 9.8, 4.6.)

  1. AIn aqueous solution, methylamine is a stronger base than ammonia
  2. BIn aqueous solution, aniline is a weaker base than ammonia
  3. CIn aqueous solution, trimethylamine is a stronger base than methylamine
  4. DIn the gas phase, trimethylamine is a stronger base than ammonia

Answer: A, B, D

A higher conjugate-acid pKa means a stronger base. Methylamine (10.6) is stronger than ammonia (9.25), so (A) is true. Aniline (4.6) is weaker than ammonia, so (B) is true. Trimethylamine (9.8) is weaker than methylamine (10.6) in water, so (C) is false. In the gas phase, basicity rises with each methyl group, so (D) is true.
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