See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Concept: The Beckmann rearrangement proceeds via the departure of the leaving group (gamma) from the nitrogen of the oxime. The rate of the Beckmann rearrangement depends on the leaving group ability of the group attached to the oxygen of the oxime (the gamma group). A better leaving group leads to a faster reaction. Step 1: Identify the leaving groups. (i) CH3CO2- (acetate): a moderately good leaving group derived from a weak acid (pKa ~4.75) (ii) Cl-CH2-CO2- (chloroacetate): a better leaving group than acetate because the electron-withdrawing Cl makes the conjugate acid (chloroacetic acid, pKa ~2.86) stronger, meaning the anion is more stable and a better leaving group. (iii) Ph-SO3- (benzenesulfonate): an excellent leaving group. Sulfonate esters are among the best leaving groups in organic chemistry because benzenesulfonic acid is a strong acid (pKa ~ -1), making the sulfonate anion highly stable. Step 2: Rank leaving group ability. Ph-SO3- >> Cl-CH2-CO2- > CH3CO2- (iii) >> (ii) > (i) Step 3: Correlate with rate. Better leaving group = faster Beckmann rearrangement. Therefore: Rate order is (iii) > (ii) > (i). Step 4: Why other options fail. (a) (i) > (ii) > (iii): Incorrect; this reverses the leaving group ability order entirely. (b) (ii) > (i) > (iii): Incorrect; sulfonate is a far superior leaving group than carboxylates. (d) (iii) > (i) > (ii): Incorrect; chloroacetate is a better leaving group than acetate due to the inductive effect of Cl, so (ii) > (i). Therefore, the correct answer is C.