See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: Grignard reagent formation from alkyl halides requires that no protic functional groups are present in the molecule. The N-H group in piperidine is a protic group (pKa ~11). When Mg is added to 3-bromopiperidine in ether, the Mg will first react with the more acidic N-H bond rather than forming the C-Mg bond at C3, because the N-H is a protic site that will protodehalogenate or prevent Grignard formation at carbon. Step 2 - Reasoning: The N-H proton in the secondary amine (piperidine) is sufficiently acidic to react with the Grignard reagent as fast as it forms, or to react with Mg preferentially. In practice, the Grignard reaction on 3-bromopiperidine (with free NH) does not give the C-Grignard; instead, Mg reacts with the N-H to give an N-MgBr species (a magnesium amide/salt). This means the carbon-Grignard is not formed, and the subsequent addition to PhCHO cannot occur at C3. Step 3 - Correct product: Since Mg reacts with the N-H proton of the piperidine nitrogen, the product after workup with H3O+ is simply the recovered or slightly modified starting amine without addition to benzaldehyde at carbon. The N-MgBr salt upon treatment with PhCHO would give N-alkylation or no reaction at carbon, and upon aqueous workup (H3O+) would regenerate the N-H. Thus the correct product obtained is the starting 3-bromopiperidine recovered, or more precisely, the reaction fails to give the Grignard adduct. The correct answer (option B) indicates that no C-Grignard addition product is formed and the correct product is the one where the N-H reacts with Mg, giving upon workup the unchanged amine (or a product where no new C-C bond is formed at C3). Option B represents the product where N-H reacts with Mg first, consuming the Grignard reagent, so no addition to PhCHO at carbon occurs, and the actual isolated product after H3O+ workup is simply 3-bromopiperidine (starting material recovered) or the amine without the benzyl alcohol substituent. Step 4 - Why the shown product fails: The shown product requires a C3-Grignard reagent from 3-bromopiperidine, but the N-H group is incompatible with Grignard formation—it destroys the Grignard reagent. Hence the alpha-hydroxybenzyl product at C3 cannot be obtained. Therefore, the correct answer is B.