See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: The Wolff rearrangement involves the photolysis or thermolysis of an alpha-diazoketone to generate a ketene intermediate via loss of N2 and a 1,2-shift. The ketene then reacts with a nucleophilic solvent. Step 2 - Mechanism with 2-diazocyclohexanone: 2-Diazocyclohexanone is an alpha-diazoketone. Upon irradiation with light, N2 is expelled to generate an alpha-oxocarbene (or an excited singlet carbene alpha to the carbonyl). This intermediate undergoes the Wolff rearrangement: the C1-C2 bond migrates to the carbene center at C2, causing ring contraction. Specifically, the carbon chain of the six-membered ring contracts by one carbon, converting the six-membered ring into a five-membered ring ketene (cyclopentylketene or more precisely, the ketene derived from ring contraction: a cyclopentane ring bearing a terminal =C=O). Step 3 - Reaction with methanol: The ketene intermediate (cyclopentane-fused or cyclopentyl ketene) reacts with methanol (a nucleophile/protic solvent) via nucleophilic addition to the ketene. Methanol adds across the C=C=O of the ketene to give the methyl ester. This produces methyl cyclopentanecarboxylate: a cyclopentane ring bearing a -CO2CH3 group. Step 4 - Product identification: Methyl cyclopentanecarboxylate corresponds to option (a): cyclopentane ring with -CO2CH3 substituent. Step 5 - Why other options fail: - Option (b): A cyclohexanone with an OCH3 group would result from simple nucleophilic substitution of the diazo group without ring contraction, not from a Wolff rearrangement. - Option (c): A diketone would require a different reaction pathway (e.g., oxidation), not the Wolff rearrangement in methanol. - Option (d): Cyclopentanecarboxylic acid (free acid) would be the product if water were the solvent, not methanol. In methanol, the ester is formed preferentially. Therefore, the correct answer is A.