See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the reagents: gamma-butyrolactone (GBL) is a five-membered cyclic ester. The Grignard reagent is a di-Grignard (Grignard reagent on both ends): BrMg-CH2CH2CH2CH2-MgBr (1,4-bis(bromomagnesio)butane). Step 2 - First Grignard addition: One end of the di-Grignard opens the lactone by attacking the carbonyl carbon of GBL. This ring-opening gives a magnesium alkoxide intermediate. The lactone C=O is attacked, the ring oxygen-carbon bond remains, giving an open-chain intermediate with a -OMgBr at one end (from the lactone oxygen) and a -CH2CH2CH2CH2MgBr tether at the other end. Step 3 - Intramolecular second addition: The remaining MgBr end of the chain now attacks the ketone-like carbonyl (now an aldehyde/ketone intermediate after first addition). Actually, more precisely: after the first attack of one MgBr on the ester carbonyl, a tetrahedral intermediate forms which collapses (esters react twice with Grignard reagents) - the first addition gives a ketone intermediate (after expulsion of the alkoxide), and immediately the second MgBr group attacks intramolecularly. The net result is an intramolecular bis-addition. Step 4 - Net result: The di-Grignard (4-carbon chain) adds to the lactone (which contributes a 4-carbon chain including the carbonyl carbon and the three CH2 groups of GBL). The intramolecular cyclization forms a five-membered carbocyclic ring (cyclopentane) with a quaternary carbon (the former carbonyl carbon) bearing two oxygen-containing substituents. After H3O+ workup, both OMgBr groups become OH groups. Step 5 - Structure of product: The former lactone carbonyl carbon becomes a quaternary carbon in a cyclopentane ring, bearing one -OH group. The oxygen from the lactone ring becomes a -OH at the end of the -(CH2)3- chain attached to that same quaternary carbon. This gives 1-(3-hydroxypropyl)cyclopentanol - i.e., a cyclopentane ring with a quaternary carbon that has an -OH directly on it and a -(CH2)3-OH chain. This matches option (b): cyclopentane with quaternary C bearing HO and -(CH2)3-OH. Step 6 - Why other options fail: - Option (a) is an open-chain keto-alcohol, not a cyclic product; ring closure does not occur in this way. - Option (c) shows two separate OH groups in a branched arrangement inconsistent with the di-Grignard cyclization mechanism. - Option (d) shows a propyl chain (-CH2CH2CH3) rather than a hydroxypropyl chain (-(CH2)3OH); the terminal OH from the lactone oxygen would be lost, which does not occur. Therefore, the correct answer is B.