Problem 58 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reagents is used to identify acetic acid? (a) Tollen's reagent (b) Neutral \(\mathrm{FeCl}_{3}\) solution (c) Ceric ammonium nitrate (d) \(\mathrm{NaOH}\) and \(\mathrm{I}_{2}\)

Step-by-Step Solution

Answer

(d) \( \mathrm{NaOH} \) and \( \mathrm{I}_{2} \) is used to identify acetic acid.

Step 1: Analyzing Tollen's Reagent

Tollen's reagent is commonly used to test for the presence of aldehydes. Since acetic acid is a carboxylic acid, not an aldehyde, Tollen's reagent is not suitable for its detection.

Step 2: Evaluating Neutral \( \text{FeCl}_{3} \) Solution

A neutral \( \text{FeCl}_{3} \) solution is used to identify phenolic compounds through a reddish-violet color change. Acetic acid does not have a phenolic group, so this reagent is not applicable for identifying acetic acid.

Step 3: Considering Ceric Ammonium Nitrate

Ceric ammonium nitrate is used for the identification of alcohols, as it shows a color change in their presence. Acetic acid does not react with this reagent as it is not an alcohol.

Step 4: Assessing \( \text{NaOH} \) and \( \text{I}_{2} \)

This combination is known as the Iodoform test, which is used for detecting the presence of methyl ketones and compounds capable of forming them. Acetic acid, being a carboxylic acid with a methyl group, can undergo this reaction, forming a yellow precipitate of iodoform \( (\text{CHI}_3) \).

Key Concepts

Tollen's Reagent

Tollen's reagent is a classical chemical solution known for detecting aldehydes due to its ability to oxidize these compounds. It consists of a solution of silver nitrate ( $AgNO_3$ ), ammonia, and in some preparations, sodium hydroxide ( $NaOH$ ). The key reaction involves the reduction of silver ions to metallic silver, which forms a mirror-like coating on the inside of the test tube. This "silver mirror" is a positive indication of aldehyde presence. However, because acetic acid is a carboxylic acid, it does not react with Tollen's reagent.

Therefore, while Tollen's reagent is a valuable tool in the identification of aldehydes, it does not serve the purpose of detecting carboxylic acids such as acetic acid.

Neutral FeCl3 Solution

A neutral $FeCl_3$ (ferric chloride) solution is applied widely for the identification of phenolic compounds. This test is known for its colorimetric response, where the presence of phenols results in a distinct reddish-violet coloration. The mechanism involves complex formation between the phenolic groups and ferric ions.

Unfortunately, acetic acid lacks the phenolic group necessary for this reaction and thus, does not produce a color change with neutral $FeCl_3$ . This makes this reagent ineffective for identifying acetic acid.

Ceric Ammonium Nitrate

Ceric ammonium nitrate functions as a test for alcohols, providing a visual cue to their presence by displaying a color change from yellow to red. This change results from a redox reaction where ceric ions ( $Ce^{4+}$ ) are reduced by the alcohol to cerous ions ( $Ce^{3+}$ ), while oxidizing the alcohol.

However, since acetic acid is not an alcohol but a carboxylic acid, this reagent does not result in a color change when introduced to acetic acid. Consequently, ceric ammonium nitrate is not suitable for detecting acetic acid.

Iodoform Test

The Iodoform test uses a combination of $NaOH$ and $I_2$ to detect methyl ketones and certain other compounds, like alcohols and carboxylic acids capable of forming methyl ketones. In this test, the substance is treated with $I_2$ and $NaOH$ under basic conditions. If the substance can generate a methyl ketone group, a yellow precipitate of iodoform ( $CHI_3$ ) will form, indicating a positive test result.

Acetic acid, though a carboxylic acid, contains a methyl group adjacent to the carbonyl, making it eligible for the reaction, resulting in a positive iodoform test. This test is crucial for confirming the presence of acetic acid and distinguishes it from other non-reactive substances.