See image — Alcohols Phenols and Ethers Chemistry Question

💡 Solution & Explanation

Concept: Esterification mechanism and isotope labeling. The reaction is between oxalic acid (HOOC-COOH) and ethylene glycol bearing 18O labels (HOCH2-CH2OH where both OH groups are 18OH) under acid catalysis with heat, forming a cyclic diester (dioxolane-2,3-dione type, i.e., the cyclic ester of oxalic acid and ethylene glycol). Step 1 – Identify the reactants: - Oxalic acid: HOOC-COOH (no isotope label) - Ethylene glycol with 18O labels on both hydroxyl groups: H-18O-CH2-CH2-18O-H Step 2 – Esterification mechanism: In acid-catalyzed esterification, the acyl-oxygen bond of the carboxylic acid breaks (i.e., the C-OH of the acid is lost as water). The oxygen from the alcohol (here 18OH) becomes the ester linkage oxygen (the -O- between carbonyl carbon and the CH2). The carbonyl oxygen (C=O) remains on the acid carbon. Step 3 – Determine which oxygens end up where: - The carboxylic acid contributes the C=O oxygen (unlabeled, 16O) and loses its -OH as water. - The alcohol contributes its 18O to form the C-18O-CH2 ester bond. - Since ethylene glycol has 18O on both -OH groups, both ester C-O-CH2 linkages will carry 18O. - The carbonyl oxygens (C=O) remain unlabeled (16O). Step 4 – Product structure: The cyclic diester formed is: O=C-(18)O-CH2 | | C-(18)O-CH2 ||O Both bridging oxygens (connecting carbonyl carbons to CH2 groups) are 18O, and both carbonyl oxygens are 16O (unlabeled). Step 5 – Match to options: - Option (a): No 18O label at all — incorrect. - Option (b): 18O on the C-O-CH2 bridging oxygens (ester oxygens), carbonyl oxygens unlabeled — this matches the mechanistic prediction. - Option (c): 18O on the carbonyl oxygens (C=O), bridging oxygens unlabeled — this would require the alcohol oxygen to become the carbonyl, which contradicts the acyl-oxygen cleavage mechanism. - Option (d): Open chain product, no cyclization — incorrect for these conditions. Why other options fail: - (a) ignores the isotope label entirely. - (c) places 18O on carbonyl positions, which would require O-alkyl cleavage mechanism (not favored under acid catalysis for primary alcohols). - (d) is an open-chain half-ester, not the expected cyclic product under these conditions. Therefore, the correct answer is B.