See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

We analyze each pair for the nitrogen inversion barrier: **Concept:** The nitrogen inversion barrier (ΔG‡) depends on the geometry at nitrogen, ring strain, electronegativity of substituents, and steric effects. The transition state for inversion is planar (sp2-like) at nitrogen. **Option (a): NMe3 vs N(i-Pr)2Me** Trimethylamine (NMe3) has a known inversion barrier of ~7.9 kcal/mol. Replacing methyl groups with bulky isopropyl groups in N(i-Pr)2Me actually lowers the inversion barrier because the bulky groups prefer the planar transition state (they spread out better in a planar geometry, relieving steric strain in the pyramidal ground state). A very low ΔG‡ of ~0.2 kcal/mol for N(i-Pr)2Me is consistent with this steric flattening effect. This data is correctly placed. **Option (b): N-methylaziridine vs N-methylpyrrolidine** N-methylaziridine has a very high inversion barrier (~20.5 kcal/mol) because the ring constrains the C-N-C angle to ~60°, and going to a planar sp2 nitrogen in the transition state would require the ring to adopt an even more strained geometry. The angle strain in the transition state is enormous. N-methylpyrrolidine (5-membered ring) has a much lower barrier (~7.0 kcal/mol) close to acyclic amines since the 5-membered ring can more easily accommodate a planar nitrogen. This data is correctly placed. **Option (c): NMe3 vs NCl3** Trimethylamine has ΔG‡ = 7.9 kcal/mol. NCl3 (trichloramine) has a dramatically higher inversion barrier (~22.9 kcal/mol) because chlorine is highly electronegative and stabilizes the pyramidal ground state through increased s-character in the nitrogen lone pair (the lone pair becomes more s-like in the pyramidal form when electronegative substituents are attached), making the pyramidal form much more stable relative to the planar transition state. This is consistent with the known trend that electronegative substituents raise the inversion barrier. This data is correctly placed. **Why (d) is correct:** All three pairs (a), (b), and (c) have correctly placed data based on well-established principles of amine inversion barriers — steric flattening in (a), ring-strain angle effects in (b), and electronegativity stabilization of pyramidal geometry in (c). Therefore, the correct answer is D.