See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Concept: When a Grignard reagent adds to a ketone that already contains a chiral center, a new chiral center is created at the carbonyl carbon. The existing chiral center is configurationally fixed (it is not racemized under these conditions), but the new chiral center formed by nucleophilic addition can be created with either Re or Si face attack, giving two possible configurations at the new center. Step 1 - Identify the starting material: The ketone shown has one existing stereocenter (the alpha carbon bearing Et, Me, H in a specific configuration as drawn). The carbonyl carbon is the site of Grignard addition. Step 2 - Grignard addition: MeMgBr adds methyl nucleophile to the carbonyl carbon. The carbonyl carbon becomes a new tetrahedral stereocenter bearing: OH, Me (from Grignard), the existing chiral carbon chain, and the ethyl chain. This creates a second chiral center. Step 3 - Stereochemical outcome: The existing chiral center remains fixed (single enantiomer starting material). The new chiral center at the former carbonyl carbon can be formed with either configuration (Re or Si face attack), giving two products. Since the two stereocenters are in the same molecule and the existing center is fixed, the two products are NOT enantiomers of each other - they are diastereomers (they differ in configuration at one center but not the other). Step 4 - Why not racemic: Racemic would mean a 50:50 mixture of enantiomers. Since the starting material is a single enantiomer (one chiral center already fixed), the two products formed have two chiral centers each, and they are diastereomers, not enantiomers. Step 5 - Why not single stereoisomer: There is no facial selectivity enforced (no chiral catalyst, no strong steric bias specified as giving only one product), so both faces of the carbonyl are attacked, giving a mixture. Step 6 - Why not meso: Meso requires identical substituents on two stereocenters with internal mirror plane; that is not the case here with Et, Me, OH, and different groups. Conclusion: The product is a mixture of two diastereomers (differing at the newly formed stereocenter, with the original stereocenter unchanged). Therefore, the correct answer is A.