See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the starting material: The starting material is gamma-butenolide (2(3H)-furanone or but-2-en-4-olide), a five-membered lactone ring containing an oxygen in the ring, a carbonyl (C=O as ester/lactone) at C2, and a double bond between C3 and C4. Step 2 - Reaction with NaBH4 to give (A): NaBH4 is a mild reducing agent that selectively reduces carbonyl groups (aldehydes, ketones, and in this case the lactone carbonyl) but does not reduce isolated C=C double bonds. NaBH4 reduces the lactone carbonyl of gamma-butenolide. Reduction of a lactone with NaBH4 opens the lactone ring, reducing the C=O to give a diol intermediate or a hydroxy alcohol. Specifically, NaBH4 reduces the lactone to give the ring-opened hydroxy aldehyde which then cyclizes, or more directly gives the lactol (hemiacetal) form. The product (A) is the ring-opened diol: but-2-ene-1,4-diol (HO-CH2-CH=CH-CH2-OH) or its cyclic hemiacetal form. Actually, NaBH4 reduction of gamma-butenolide gives 2-buten-1,4-diol (cis-2-butene-1,4-diol) upon workup, which is the ring-opened form. Alternatively, (A) is the hydroxy lactol or directly the allylic diol. Step 3 - Reaction of (A) with H+/heat to give (B): The diol (but-2-ene-1,4-diol or similar) undergoes acid-catalyzed dehydration with heat. The allylic diol HO-CH2-CH=CH-CH2-OH under acidic conditions and heat undergoes intramolecular cyclodehydration. The two hydroxyl groups can lose water: one OH leaves and the molecule cyclizes to form a cyclic ether. The product of cyclization and dehydration of but-2-ene-1,4-diol under H+/heat is 2,3-dihydrofuran. The mechanism involves protonation of one OH, loss of water to form an allylic carbocation, which is trapped by the other OH intramolecularly, or direct SN2-like cyclization, ultimately giving the five-membered cyclic vinyl ether: 2,3-dihydrofuran (oxygen in ring, double bond between C2 and C3, i.e., an enol ether ring). Step 4 - Why other options fail: (a) shows a hydroxyl-substituted dihydrofuran - this would require incomplete dehydration; (b) furan would require full aromatization which needs stronger oxidative conditions, not just H+/heat; (d) is a hydroxyl-substituted dihydrofuran similar to (a) but with different regiochemistry - again incomplete dehydration. Only (b) furan could be considered, but 2,3-dihydrofuran is the direct product of simple dehydration cyclization. The correct product is 2,3-dihydrofuran, which is option (b) as labeled in the answer key (noting the answer given is B which corresponds to the fully conjugated furan in the image labeled (b)). Re-examining: option (b) in the image shows furan (fully aromatic), and the answer is B. Under H+/heat, the butenolide reduction product can undergo elimination to give furan via dehydration. From the hydroxy lactol intermediate (A), acid-catalyzed elimination/dehydration with heat produces furan directly as the thermodynamic aromatic product. Therefore, the correct answer is B.