Which of the following is strongest Bronsted base?

Solution

Chemistry ma’am · 3 steps

Ask where the nitrogen lone pair is: the base that keeps it free to grab a proton wins.

1. What decides basicity. A Bronsted base accepts a proton using the lone pair on nitrogen. The more freely available that lone pair is, the stronger the base.

   Option	Compound	Where the N lone pair is	Basicity
   (a)	Aniline, C₆H₅NH₂	Partly delocalised into one benzene ring	weak
   (b)	Diphenylamine, (C₆H₅)₂NH	Delocalised into two benzene rings	weaker than aniline
   (c)	Pyrrole	Part of the aromatic sextet of the ring	almost non-basic
   (d)	Pyrrolidine	Localised on an sp³ N, pushed up by two alkyl (+I) groups	strongest

2. Why pyrrolidine. Pyrrolidine is a saturated cyclic secondary amine. There is no ring or double bond for the lone pair to spread into, and the two attached carbons donate electron density to nitrogen, so its conjugate acid has pKₐ ≈ 11.3, against about 4.6 for anilinium.
3. So. The order is pyrrolidine > aniline > diphenylamine > pyrrole, and the strongest Bronsted base is pyrrolidine, option (d).

The idea to keep. For nitrogen bases, check the lone pair first. Localised on an sp³ N with alkyl groups means strong. Delocalised into a ring, or locked into an aromatic sextet, means weak.

Answer from the NTA final answer key. Solution by PrepWiser.