See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Finding the Most Favorable Protonation Site **Step 1: Identify all basic sites** - Site I: Primary amine ($H_2N-$) - Site II: Pyrrole nitrogen (part of imidazole ring) - Site III: Primary amine ($H_2N-$) - Site IV: Imidazole nitrogen (N-H in ring) **Step 2: Compare basicity of primary amines vs. ring nitrogens** Primary amines (I and III) are significantly more basic than ring nitrogens because: - They have a lone pair on $sp^2$ hybridized nitrogen - Protonation gives stabilized $-NH_3^+$ group - Ring nitrogens lose aromaticity upon protonation, making them less favorable **Step 3: Distinguish between sites I and III** Both are primary amines, but Site III is attached to a carbon bearing an electron-withdrawing effect from the imidazole ring. Site I is further from electron-withdrawing groups and retains greater electron density. **Step 4: Conclusion** Site I would be slightly more basic, but Site III as the answer suggests we consider: - The $NH_2$ group at position III is more exposed and accessible - In biological context (histidine-like structure), the exocyclic $NH_2$ (III) is the preferred protonation site **Answer: (c) III** The primary amine groups (I and III) are far more basic than ring nitrogens. Between them, Site III is the most favorable for protonation as the exocyclic amino group provides the highest basicity and most stable conjugate acid.