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 a qualitative test used in organic chemistry to determine the presence of an aldehyde group. It works by oxidizing aldehydes to carboxylic acids while reducing the Tollens' reagent (ammoniacal silver nitrate) to metallic silver, which appears as a silver mirror on the test tube.

Let's analyze each compound: 

• Phenylethanal (C₈H₈O) contains an aldehyde group. It can give a positive Tollens test.
• Acetophenone (C₈H₈O) is a ketone, not an aldehyde. Ketones do not give a positive Tollens test.
• Formic acid (HCOOH) is not an aldehyde; it's a carboxylic acid. Even though formic acid has a structure analogous to an aldehyde, it is generally not tested with Tollens' test like typical aldehydes. It is a unique case where the reactivity towards Tollens' may appear positive due to its reductive capabilities but it is categorized separately from typical aldehyde tests.
• Benzene carbaldehyde (C₇H₆O) is another name for benzaldehyde. It is an aromatic aldehyde and gives a positive Tollens test.

From the analysis, the compounds that yield a positive Tollens test are Phenylethanal and Benzene carbaldehyde. Acetophenone, being a ketone, doesn't, and Formic acid's results are not considered typical for aldehyde identification.

Thus, the correct answer is: A, C, and D only

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.