See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: The rate of N-methylation (quaternization) with methyl iodide (an SN2 reaction) depends on the nucleophilicity of the amine nitrogen, which is influenced by steric hindrance and lone pair availability. Step 1 - Analyze structure A (triethylamine, NEt3): Triethylamine is an acyclic tertiary amine. Although it is a tertiary amine with good basicity, it suffers from significant steric hindrance due to the three ethyl groups surrounding the nitrogen. The ethyl groups can adopt conformations that partially shield the nitrogen lone pair. Additionally, in acyclic tertiary amines, the nitrogen lone pair is in a pyramidal geometry but the approach of the electrophile is hindered by the three bulky ethyl chains. This makes it the least reactive of the three toward SN2 methylation. Step 2 - Analyze structure B (quinuclidine, 1-azabicyclo[2.2.2]octane): Quinuclidine is a bridged bicyclic tertiary amine where the nitrogen is at the bridgehead. The three ethylene bridges hold the alkyl groups rigidly away from the nitrogen, completely eliminating the B-strain (back-strain) that plagues acyclic tertiary amines. The lone pair on nitrogen is perfectly exposed and unhindered from the front face (no back-side steric interaction from the bridges because they fan away). This makes quinuclidine an exceptionally good nucleophile - it reacts faster with methyl iodide than triethylamine by a large factor (approximately 30-fold or more). Quinuclidine is the most reactive. Step 3 - Analyze structure C (DABCO, 1,4-diazabicyclo[2.2.2]octane): DABCO has two nitrogen atoms in a cage structure. Like quinuclidine, the bridged bicyclic framework holds the substituents away from nitrogen, providing low steric hindrance. However, once one nitrogen is methylated (quaternized), the resulting ammonium group on the other end strongly withdraws electron density through the cage, reducing the nucleophilicity of the second nitrogen. But for the first methylation event, DABCO is less reactive than quinuclidine because: (a) the second nitrogen in DABCO is slightly electron-withdrawing through induction even before quaternization, and (b) the cage is slightly more constrained. DABCO reacts faster than triethylamine (due to reduced steric hindrance) but slower than quinuclidine. So C is intermediate. Step 4 - Establish order: B (quinuclidine) > C (DABCO) > A (triethylamine) This corresponds to answer option (c): B > C > A. Why other options fail: - (a) A > B > C: Incorrect; triethylamine is most hindered, not least. - (b) A > C > B: Incorrect; same reasoning. - (d) B > A > C: Incorrect; triethylamine is more hindered than DABCO. Therefore, the correct answer is C.