Question

Which of the following is the strongest Bronsted base?

• A.
• B.
• C.
• D.

Hint:

The strongest Bronsted base has a localized lone pair with minimal electron-withdrawing effects.

Answer 1

The Correct Option is D

Step 1: Understanding Bronsted Bases 
A Bronsted base is a substance that can accept protons. The strength of a Bronsted base depends on: 1. The availability of a lone pair for protonation. 2. The electron-donating or withdrawing effects of the surrounding groups. 
Step 2: Evaluating Each Structure 
Structure 1 has electron-withdrawing effects, reducing basicity. 
Structure 2 has delocalization of the lone pair, reducing its basicity.
Structure 3 is in a constrained ring, affecting electron availability. 
Structure 4 has a localized lone pair, making it the most basic. 
Thus, the correct answer is (D).

Answer 2

The given compound in the image is pyrrolidine — a five-membered saturated heterocyclic amine with the formula C₄H₉N. It consists of a nitrogen atom as part of a five-membered ring and an attached hydrogen atom, making it a secondary amine.

Bronsted bases are proton acceptors. The strength of a Bronsted base depends on the availability of the lone pair on the nitrogen atom for protonation.

Let us analyze why pyrrolidine is the strongest Bronsted base among common nitrogen-containing compounds:

1. Electron Density on Nitrogen: In pyrrolidine, the nitrogen is sp³-hybridized, which means its lone pair is held in an orbital with more s-character (25%) than sp² or sp hybrid orbitals. This makes the lone pair more available for protonation, enhancing basicity.

2. Non-Aromatic Structure: Pyrrolidine is not aromatic, so the lone pair on nitrogen is not involved in maintaining aromaticity. Therefore, it is free and readily available for accepting a proton.

3. Inductive Effects: The saturated ring does not strongly withdraw electrons from the nitrogen. Hence, the electron density remains high on nitrogen, increasing its basic strength.

Compare this with aromatic amines (e.g., pyridine or aniline), where the lone pair on nitrogen is either part of an aromatic system or delocalized into the ring, making it less available for protonation and thereby reducing their basicity.

Conclusion: Pyrrolidine has a highly available lone pair on nitrogen, not involved in aromaticity, and experiences minimal electron-withdrawing effects. Hence, it is the strongest Bronsted base among the options typically given.

Correct Answer: Pyrrolidine (shown in the image)