Question
Identify the compound: an organic substance reacts with Tollens’ reagent to give a silver mirror, and also reacts with HCN to give a cyanohydrin, but does NOT react with sodium bicarbonate. The compound has molecular formula . What is it, and explain each test.
Solution — Step by Step
. Possible structures: ethanol (), ethylene oxide, or acetaldehyde ().
Tollens’ reagent () oxidises aldehydes to carboxylic acids and itself reduces to silver mirror. Ethanol and ethylene oxide do not have , so they fail. Acetaldehyde passes.
So the compound contains an aldehyde group.
Aldehydes and ketones add HCN to form cyanohydrins via nucleophilic addition. Acetaldehyde passes.
reacts with carboxylic acids to release . Acetaldehyde does not react — confirming it is NOT a carboxylic acid.
The compound is acetaldehyde ().
Why This Works
Tollens’ is selective for aldehydes (not ketones, because ketones lack the -hydrogen on carbonyl required for oxidation in mild conditions). HCN addition works for both aldehydes and ketones. Bicarbonate test detects the acidic group.
The combination of these three tests narrows the structure quickly: aldehyde present, carboxylic acid absent — the only fit for is acetaldehyde.
Alternative Method
Degree of unsaturation: has DoU = . One double bond or ring. Among the candidates with , only acetaldehyde fits — its accounts for the one DoU.
Common Mistake
Confusing Tollens’ test with Fehling’s test. Both detect aldehydes, but aromatic aldehydes (like benzaldehyde) pass Tollens’ but FAIL Fehling’s. NEET 2023 had a question that explicitly tested this distinction.