Question

Which of the following gives a positive Tollens test?
(A) Phenylethanal
(B) Acetophenone
(C) Formic acid
(D) Benzene carbaldehyde
Choose the correct answer from the options given below:

• A. A, B, and D only
• B. A, B, and C only
• C. A, B, C, and D
• D. A, C, and D only

Answer 1

The Correct Option is A

The Tollens test is used to detect aldehydes, as they reduce Tollens' reagent to metallic silver. Phenylethanal and benzene carbaldehyde are aldehydes and will give a positive Tollens test. Formic acid, though a carboxylic acid, can also react with Tollens' reagent to give a positive test. Acetophenone is a ketone and does not give a positive Tollens test.

Answer 2

The Tollens' test is used to detect aldehydes, as they reduce Tollens' reagent to metallic silver, forming a characteristic silver mirror on the inside of the test tube. This reaction occurs because aldehydes are easily oxidized, while ketones are generally resistant to oxidation under these conditions.

Phenylethanol and benzene carbaldehyde are aldehydes and will give a positive Tollens' test due to the presence of the aldehyde group, which reduces the Tollens' reagent to metallic silver.

Formic acid, though a carboxylic acid, can also react with Tollens' reagent to give a positive result. This occurs because formic acid can decompose to form formaldehyde, which is an aldehyde and reacts with Tollens' reagent.

Acetophenone, a ketone, does not give a positive Tollens' test, as ketones are less reactive toward oxidation than aldehydes. The ketone group does not reduce the Tollens' reagent, so no silver mirror is formed.

In conclusion, aldehydes, such as phenylethanol and benzene carbaldehyde, give a positive Tollens' test, while ketones like acetophenone do not. Even carboxylic acids, under certain conditions like formic acid, can give a positive test.