Question

Statement-I: All aldehydes and ketones give positive Tollens' test. Statement-II: Only aldehydes give positive Fehling's test.

• A. Both I and II
• B. Neither I nor II
• C. Only II
• D. Only I

Hint:

Tollens' reagent gives a silver mirror with aldehydes, while Fehling's solution gives a brick-red precipitate mainly with aliphatic aldehydes.

Answer 1

The Correct Option is C

Concept: Tollens' reagent and Fehling's solution are important qualitative reagents used for distinguishing aldehydes and ketones. Tollens' reagent contains diamminesilver(I) ion: \[ [Ag(NH_3)_2]^+ \] which acts as a mild oxidizing agent. Fehling's solution contains alkaline copper(II) ions which are reduced to brick-red cuprous oxide in the presence of suitable reducing agents. Aldehydes are generally more easily oxidized than ketones because they contain a hydrogen atom attached directly to the carbonyl carbon.

Step 1: Examine Statement-I. Statement-I says: \[ \text{All aldehydes and ketones give positive Tollens' test} \] This statement is incorrect. Aldehydes readily reduce Tollens' reagent and produce a silver mirror. However, ordinary ketones do not reduce Tollens' reagent. Only a few special ketones such as \(\alpha\)-hydroxy ketones may give positive Tollens' test. Therefore: \[ \boxed{\text{Statement-I is false}} \]

Step 2: Examine Statement-II. Statement-II says: \[ \text{Only aldehydes give positive Fehling's test} \] Fehling's solution is reduced by aliphatic aldehydes giving a brick-red precipitate of cuprous oxide. Most ketones do not respond to Fehling's test. Therefore: \[ \boxed{\text{Statement-II is true}} \]

Step 3: Choose the correct option. Statement-I is false. Statement-II is true. Hence, \[ \boxed{\text{Only II}} \] Therefore, the correct option is \[ \boxed{(C)} \]