See image — Practical Organic Chemistry and Purification Chemistry Question

💡 Solution & Explanation

Concept: Tollen's reagent (ammoniacal silver nitrate) gives a positive test (silver mirror) with aldehydes, which are oxidized to carboxylic acids. Among carboxylic acids, only those that can be further oxidized — i.e., those that contain a hidden aldehyde-like C-H on the carbonyl carbon — give a positive Tollen's test. Step 1: Identify the structures. (a) (CH2)2(CO2H)2 is glutaric acid (pentanedioic acid) — a simple dicarboxylic acid with no oxidizable aldehyde functionality. (b) CH2(CO2H)2 is malonic acid — a simple dicarboxylic acid with no aldehyde functionality. (c) The vertical structure CO2H | CO2H represents oxalic acid (ethanedioic acid) — a dicarboxylic acid with no aldehyde functionality. (d) HCO2H is formic acid (methanoic acid). Step 2: Analyze formic acid (option d). Formic acid has the structure H-C(=O)-OH. Unlike other carboxylic acids, its carbonyl carbon bears a hydrogen atom (H-C=O), giving it both acid and aldehyde character. This means formic acid can be oxidized further by Tollen's reagent — the H on the carbonyl is oxidized, producing CO2 and water, while Ag+ is reduced to Ag(s), forming the silver mirror. Step 3: Why other options fail. Options (a), (b), and (c) are all carboxylic acids whose carbonyl carbons bear no hydrogen atoms (they are bonded to -OH and -C groups). They cannot be oxidized by Tollen's reagent and give a negative test. Step 4: Conclusion. Only HCO2H (formic acid) gives a positive Tollen's test among the given options because it possesses an aldehydic hydrogen on its carbonyl carbon, allowing it to act as a reducing agent. Therefore, the correct answer is D.