See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Concept: When a Grignard reagent adds to a ketone that already bears a chiral centre, the reaction creates a new chiral centre at the carbonyl carbon. The two faces of the carbonyl are diastereotopic (not enantiotopic) because the molecule already contains a pre-existing stereocentre. Step 1 – Identify the starting material. The starting material is (R)-3-methylcyclohexan-1-one. C3 is a stereocentre (methyl on wedge = R configuration). C1 is the carbonyl carbon. Step 2 – Grignard addition. CH3MgBr adds a methyl group to the carbonyl carbon (C1). This converts C1 from sp2 to sp3, creating a new stereocentre at C1. The product is 1-methyl-3-methylcyclohexan-1-ol (1,3-dimethylcyclohexan-1-ol). Step 3 – Analyse the two possible products. Attack from the two faces of the carbonyl gives two products: - (1R,3R)-1,3-dimethylcyclohexan-1-ol - (1S,3R)-1,3-dimethylcyclohexan-1-ol Both products retain the fixed R configuration at C3 (the original stereocentre is not destroyed). Only the configuration at C1 differs. Therefore, the two products differ at one stereocentre while sharing the other — they are diastereomers. Step 4 – Rule out other options. (a) Racemic mixture: A racemic mixture consists of a pair of enantiomers in equal amounts. Enantiomers require all stereocentres to be inverted. Here C3 remains R in both products, so they are NOT enantiomers; hence no racemic mixture. (c) Meso forms: Meso compounds have internal planes of symmetry that render them achiral despite stereocentres. Neither product here is a meso compound, and a meso compound is a single compound, not a description of two products. (d) Optically inactive due to absence of chiral centre: Both products have two chiral centres, so this is incorrect. Step 5 – Conclusion. Because the starting ketone already has one stereocentre (C3, R), the carbonyl's two faces are diastereotopic. Addition of CH3MgBr produces two diastereomeric alcohols: (1R,3R) and (1S,3R)-1,3-dimethylcyclohexan-1-ol. These are diastereomers. Therefore, the correct answer is B.