See image — Alcohols Phenols and Ethers Chemistry Question

💡 Solution & Explanation

Step 1 - Identify compound A from question 4: Based on the answer choices, compound A is a cyclohexane derivative bearing a hydroxymethyl group (-CH2OH) at C1, a methyl ester group (-C(=O)-O-CH3) at C3, and a ketone (C=O) at C5. This is consistent with option (a) being the starting material with the ketone still unreduced, and the other options representing various reduction products. Step 2 - Understand NaBH4 selectivity: NaBH4 is a mild reducing agent that selectively reduces ketones and aldehydes (carbonyl groups) to alcohols. Critically, NaBH4 does NOT reduce esters under normal conditions. It also does not affect existing alcohols. Step 3 - Apply NaBH4 to compound A: Compound A contains (i) a ketone at C5, (ii) a methyl ester at C3, and (iii) a hydroxymethyl group at C1. NaBH4 will reduce only the ketone at C5 to a secondary alcohol (-OH), leaving the methyl ester intact. Step 4 - Determine the product: After reduction of the C5 ketone, the product is a cyclohexane ring with a CH2OH group at C1, an intact -C(=O)-O-CH3 ester at C3, and a new -OH (secondary alcohol) at C5. This matches option (b). Step 5 - Eliminate other options: (a) still has the unreduced ketone - no reaction shown, incorrect. (c) has three alcohol groups with no ester - this would require ester reduction, which NaBH4 cannot do. (d) shows an aldehyde instead of an ester and has only two OH groups - structurally inconsistent with selective ketone reduction. Therefore, the correct answer is B.