Read the following passage and answer the questions based on it.

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two large groups in ketones hinders the attack of nucleophile to carbonyl carbon than in aldehydes. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in the former.

Which of the following will respond to Tollen's test?

Methanoic acid

The correct answer is Option (2) → Methanoic acid.

Tollen's reagent is a solution of silver nitrate (\( \text{AgNO}_3 \)) in ammonia (\( \text{NH}_3 \)). The silver ions (\( \text{Ag}^+ \)) in the reagent can be reduced to metallic silver (\( \text{Ag} \)) by certain types of organic compounds.

Tollen's test is primarily used to identify aldehydes, as they can reduce the silver ions to metallic silver, producing a characteristic "silver mirror" effect on the surface of the test tube.

How Tollen's Test Works

Reaction Mechanism:

When an aldehyde is mixed with Tollen's reagent, the carbonyl group (\( \text{C=O} \)) in the aldehyde undergoes oxidation, while the silver ions (\( \text{Ag}^+ \)) in Tollen's reagent are reduced to metallic silver. The general reaction can be summarized as follows:

\(\text{RCHO} + \text{Ag}^+ \rightarrow \text{RCOOH} + \text{Ag} \downarrow\)

The silver precipitate forms as a shiny mirror on the glass surface.

Tollen's test is performed under mild conditions, and it is specifically sensitive to aldehydes and some reducing sugars. Ketones, however, do not typically respond to this test because they lack the necessary hydrogen atom attached to the carbonyl carbon that is found in aldehydes.

Analysis of Each Compound

Now let us examine each of the four compounds you provided:

1. Ethanoic Acid (\( \text{CH}_3\text{COOH} \)):

Type: Carboxylic acid.

Structure: Contains a carbonyl group (\( \text{C=O} \)), but it is part of a carboxyl group (\( \text{–COOH} \)).

Response: No, because carboxylic acids do not reduce Tollen's reagent. The presence of the hydroxyl group in the carboxylic acid makes it less reactive in this context, as they typically do not behave like aldehydes in terms of reducing agents.

2. Methanoic Acid (\( \text{HCOOH} \)):

Type: Carboxylic acid (also known as formic acid).

Structure: The simplest carboxylic acid, with a carbonyl group directly attached to a hydrogen atom.

Response: Yes, methanoic acid can reduce Tollen's reagent. It behaves similarly to an aldehyde in this reaction because it can be oxidized to carbon dioxide, thus allowing it to donate electrons to the silver ions. The reaction can be summarized as:

\(\text{HCOOH} + \text{Ag}^+ \rightarrow \text{CO}_2 + \text{H}^+ + \text{Ag}\)

This property is unique to methanoic acid among the carboxylic acids.

3. Propanoic Acid (\( \text{C}_2\text{H}_5\text{COOH} \)):

Type: Carboxylic acid.

Structure: Similar to ethanoic acid, with a longer carbon chain.

Response: No, it does not respond to Tollen's test. Like ethanoic acid, it cannot reduce Tollen's reagent.

4. Butanoic Acid (\( \text{C}_3\text{H}_7\text{COOH} \)):

Type: Carboxylic acid.

Structure: Contains a longer carbon chain and, like the others, has a carboxyl group.

Response: No, as with propanoic and ethanoic acids, it does not reduce Tollen's reagent.

Conclusion

In summary, Tollen's test is effective for identifying aldehydes, and while methanoic acid is a carboxylic acid, its unique structure allows it to reduce Tollen's reagent. Therefore, it responds positively to the test. The other carboxylic acids do not exhibit this reactivity. Thus, the only correct answer from the options provided is: Methanoic Acid (HCOOH).