See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: Decarboxylation of β-keto carboxylate anions proceeds via a cyclic transition state in which the carboxylate donates electrons to form a C=C (enolate), releasing CO2. The rate of decarboxylation depends on the stability of the transition state and the resulting carbanion/enolate intermediate. Electron-withdrawing groups (EWGs) on the α-carbon stabilize the developing negative charge in the transition state, thereby accelerating decarboxylation. Step 2 - Reasoning: The substituents on the α-carbon in these four compounds are F, Cl, NO2, and CN. The ability of these groups to stabilize a carbanion/negative charge through induction and/or resonance determines the rate of decarboxylation. - NO2 is a powerful electron-withdrawing group both by induction (-I) and by resonance (-M), providing exceptional stabilization of the negative charge developing at the α-carbon in the transition state. - CN is a strong EWG by induction and by resonance (through the π* of the nitrile), also providing very good stabilization. - Cl is an EWG by induction but a weak +M donor, providing moderate stabilization. - F is a weak EWG by induction (despite high electronegativity, it is less effective at stabilizing carbanions inductively over distance) and a +M donor (electron donation by lone pairs into the ring/adjacent system), providing the least stabilization. Step 3 - Order of decarboxylation rate (fastest to slowest): NO2 (c) > CN (d) > Cl (b) > F (a). This means compound (c) with NO2 undergoes decarboxylation most readily, making it the correct answer as the one with the highest rate / correct order designation. Step 4 - Why other options fail: Option (a) F is a poor carbanion stabilizer due to its +M effect overriding induction at the α-carbon. Option (b) Cl is intermediate. Option (d) CN is strong but NO2 is stronger due to its combined −I and −M effects being superior. The question asks for the correct order, and the answer is centered on (c) NO2 being the fastest/most favorable. Therefore, the correct answer is C.