See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: The target compound (A) is a bicyclic alpha,beta-unsaturated lactone (specifically a hexahydrobenzofuran-2-one with a double bond between C3 and C3a, bearing a CH3 at C3). This type of alpha,beta-unsaturated lactone can be formed by an E2 elimination (or base-induced dehydrohalogenation) of an alpha-halo lactone using EtO- (ethoxide base). Step 1 - Identify the elimination requirement: To form the C3=C3a double bond in the product, we need a precursor that has a leaving group (Br) at C3 and an H at C3a (or vice versa, Br at C3a and H at C3), positioned in an antiperiplanar arrangement for E2 elimination. Step 2 - Analyze option (a): Option (a) shows the bicyclic lactone with H on a wedge at C3a (ring junction) and Br on a dash at C3 (along with CH3 on a dash at C3). The H at C3a and Br at C3 are on opposite faces (anti relationship) due to the trans-fused ring system stereochemistry shown. EtO- abstracts the H at C3a while Br leaves from C3 in an E2 fashion, generating the C3-C3a double bond and retaining the CH3 at C3, giving exactly compound (A). Step 3 - Why other options fail: - Option (b): The stereochemistry of H at C3a and Br at C3 is different (both on same face/syn relationship), making E2 elimination geometrically disfavored or giving a different product. - Option (c): The structure has a quaternary carbon (no H on the carbon bearing Br and CH3), so E2 elimination toward C3=C3a is not possible in the same manner; also the ring system appears different (possibly spiro). - Option (d): The configuration at the alpha carbon and ring junction differs such that the required anti-periplanar H-C-C-Br arrangement for E2 is not present, or elimination would yield a different regiochemical/stereochemical outcome. Step 4 - Conclusion: Only option (a) provides the correct stereochemical relationship (anti H and Br at adjacent carbons C3a and C3) needed for E2 elimination with EtO- to produce the target alpha,beta-unsaturated lactone (A) with the CH3 substituent. Therefore, the correct answer is A.