In organic chemistry, the classical Silver Mirror (Tollens) test is given by which of the following carbonyl compounds: 1. Benzophenone, 2. Acetaldehyde, 3. Acetone, 4. Formaldehyde?

Introduction / Context:
This question tests your understanding of the Tollens test, also called the Silver Mirror test, in organic chemistry. The test is used to distinguish aldehydes from ketones based on their ability to be oxidised by Tollens reagent. Recognising which functional groups give a positive Silver Mirror test is essential when identifying unknown carbonyl compounds in qualitative organic analysis.

Given Data / Assumptions:
- Four carbonyl compounds are listed: benzophenone, acetaldehyde, acetone and formaldehyde.
- Tollens reagent (ammoniacal silver nitrate) is used in the Silver Mirror test.
- A positive test is indicated by the deposition of a shining silver mirror on the inner surface of the reaction vessel.
- We assume standard laboratory conditions and definitions where typical aldehydes are easily oxidised, whereas simple ketones are not.

Concept / Approach:
Tollens reagent is a mild oxidising agent that oxidises aldehydes to corresponding carboxylic acids while itself being reduced to metallic silver. Ordinary ketones are generally not oxidised by such a mild reagent and therefore do not give the Silver Mirror. Among the given compounds, acetaldehyde and formaldehyde are aldehydes, while benzophenone and acetone are ketones. Therefore, acetaldehyde and formaldehyde should give a positive Tollens test and produce the silver mirror.

Step-by-Step Solution:
Step 1: Identify the functional group in benzophenone. It contains a carbonyl group attached to two phenyl rings (a diaryl ketone), so it is a ketone. Step 2: Identify the functional group in acetaldehyde (CH3CHO). It has a terminal aldehyde group, so it is an aldehyde. Step 3: Identify the functional group in acetone (CH3COCH3). It is a simple dialkyl ketone, not an aldehyde. Step 4: Identify the functional group in formaldehyde (HCHO). It is the simplest aldehyde with a formyl group. Step 5: Apply Tollens test rule: aldehydes (acetaldehyde and formaldehyde) are oxidised and give the Silver Mirror, while ordinary ketones (benzophenone and acetone) typically do not. Step 6: Conclude that compounds 2 and 4 give a positive Silver Mirror test, so the correct option is 2 and 4.

Verification / Alternative check:
In the presence of Tollens reagent, acetaldehyde is oxidised to acetic acid, and formaldehyde is oxidised to formic acid. During these redox reactions, silver ions are reduced to metallic silver, producing the characteristic mirror. On the other hand, acetone and benzophenone resist oxidation by this mild reagent because the carbonyl carbon in ketones is less easily oxidised without breaking carbon–carbon bonds. This difference in reactivity between aldehydes and ketones is a standard result in qualitative organic chemistry and confirms that compounds 2 and 4 are correct.

Why Other Options Are Wrong:
Only 1: Benzophenone is a ketone and does not normally give a Silver Mirror, so this option is incorrect.
Only 2: Acetaldehyde does give the Silver Mirror, but formaldehyde also does, so 2 alone is incomplete.
1 and 3: Both benzophenone and acetone are ketones and typically do not respond positively to Tollens reagent.
Any option that excludes formaldehyde or acetaldehyde is inconsistent with the known behaviour of aldehydes.

Common Pitfalls:
Students sometimes think that all carbonyl compounds respond similarly to Tollens reagent. Another common mistake is to confuse Fehling test and Tollens test or to forget that many ketones require stronger oxidising agents for further reaction. To avoid confusion, remember that Tollens reagent is specifically used for aldehydes and that formaldehyde, being very easily oxidised, gives one of the strongest Silver Mirror responses.

Final Answer:
The Silver Mirror (Tollens) test is given by acetaldehyde and formaldehyde, that is compounds 2 and 4.