See image — Alcohols Phenols and Ethers Chemistry Question

💡 Solution & Explanation

Step 1 - Identify what the reaction does: Esterification involves only the carbonyl carbon (the carboxylic acid carbon). The problem statement explicitly states this. Therefore, the chiral center (the alpha-carbon bearing the OH group) is NOT touched during the reaction. Step 2 - Absolute configuration vs. optical rotation: The absolute configuration (R or S) of the chiral center is determined by the spatial arrangement of substituents and their CIP priorities. The sign of optical rotation (+/-) is an experimentally measured property that depends on how a compound interacts with plane-polarized light. Crucially, there is NO direct, predictable relationship between absolute configuration (R/S) and the sign of optical rotation (+/-). A compound with R configuration can be (+) or (-), and vice versa. Step 3 - What happened here: The chiral center was not disturbed (no bond at the chiral center was broken or formed). The absolute configuration (R or S) remained the same. However, when the -COOH group is converted to -COOCH3, the CIP priorities of the substituents around the chiral center may change, potentially changing the R/S designation on paper - but even setting that aside, the sign of optical rotation changed from (-) to (+). This change in optical rotation sign occurred simply because the overall molecule changed (different substituents affect the interaction with polarized light), not because of any inversion at the chiral center. Step 4 - Why other options fail: (a) S_N2 at the chiral center is wrong because the reaction involves only the carbonyl carbon, not the chiral center. No inversion occurred at the stereocenter. (b) S_N1 at the chiral center is wrong for the same reason - no reaction occurred at the chiral center. (c) No diastereomer was produced; the chiral center configuration was unchanged and only one stereoisomer is present. Diastereomers require two or more stereocenters with different configurations. Step 5 - Why D is correct: The sign of optical rotation changed from (-) to (+) not because the absolute configuration changed, but because the optical rotation of a compound is an empirical property not directly predictable from absolute configuration. Changing -COOH to -COOCH3 alters the molecule's interaction with polarized light, giving a different sign of rotation - this is coincidental with respect to absolute configuration, as optical rotation sign and R/S designation have no fixed correspondence. Therefore, the correct answer is D.