Benzaldehyde gives a positive test with which of the following?

a.) Tollen's reagent

b.) Fehling solution

c.) Benedict's solution

d.) All of these

Benzaldehyde, which is an aromatic aldehyde, reacts positively with Tollen's reagent. This reagent is specifically designed to test for aldehydes, and it can oxidize benzaldehyde to benzoic acid, resulting in a silver mirror formation on the test tube. Let's break down the options to clarify why benzaldehyde does not give a positive test with Fehling's solution or Benedict's solution.

Understanding the Reagents

To determine the correct answer, it's essential to understand how each reagent works:

• Tollen's Reagent: This is a solution of silver nitrate in ammonia. It is used to detect aldehydes, which can be oxidized to carboxylic acids. When benzaldehyde is present, it reduces the silver ions to metallic silver, creating a characteristic silver mirror.
• Fehling's Solution: This consists of two solutions (Fehling's A and B) that, when mixed, form a deep blue solution containing copper(II) ions. Fehling's solution is used to test for reducing sugars and aldehydes, but it is more effective with aliphatic aldehydes. Benzaldehyde does not react with Fehling's solution because it is less reactive than aliphatic aldehydes.
• Benedict's Solution: Similar to Fehling's, Benedict's solution tests for reducing sugars and aldehydes. It contains copper(II) sulfate and is also more effective with aliphatic aldehydes. Benzaldehyde does not produce a positive result with Benedict's solution for the same reasons as with Fehling's.

Conclusion on the Options

Given this information, the only reagent that benzaldehyde gives a positive test with is Tollen's reagent. Therefore, the correct answer to your question is:

a.) Tollen's reagent

Why the Other Options Fail

To further clarify, let's consider why benzaldehyde does not react with the other two solutions:

• Fehling's and Benedict's solutions are designed to react with more reactive aldehydes or reducing sugars. Benzaldehyde's aromatic structure makes it less reactive compared to aliphatic aldehydes, which is why it does not produce a positive result.
• In practical laboratory settings, if you were to test benzaldehyde with Fehling's or Benedict's solution, you would observe no color change, indicating no reaction has occurred.

In summary, when testing for benzaldehyde, Tollen's reagent is the go-to choice, while Fehling's and Benedict's solutions are ineffective due to the nature of benzaldehyde's reactivity. This distinction is crucial in organic chemistry, especially when identifying functional groups in various compounds.