Problem 110 - A Complete Resource Book in Chemistry for JEE Main

Question

Phenol and benzoic acid can be distinguished by which tests? (1) Libermann nitroso test (2) Reaction with \(\mathrm{NaHCO}_{3}\) (3) Reaction with \(\mathrm{NaOH}\) (4) Ferric chloride test (a) 1,2 and 3 (b) 1,2 and 4 (c) 1,3 and 4 (d) 2,3 and 4

Step-by-Step Solution

Answer

(b) 1,2 and 4

Step 1: Understand Test Reactions for Phenol

Phenol reacts with \(\text{FeCl}_3\) to produce a violet color, indicating a positive reaction for phenol in the Ferric chloride test. It does not react with \(\text{NaHCO}_3\), and with NaOH, phenol forms a salt known as sodium phenoxide.

Step 2: Understand Test Reactions for Benzoic Acid

Benzoic acid reacts with \(\text{NaHCO}_3\) to produce carbon dioxide gas due to its acidic nature, indicating a positive reaction for benzoic acid. It also reacts with NaOH to form sodium benzoate, but does not undergo the Ferric chloride test.

Step 3: Libermann Nitroso Test

The Libermann nitroso test is typically used for detecting phenols. Benzoic acid does not react to this test.

Step 4: Compare Tests to Distinguish Phenol and Benzoic Acid

To distinguish phenol from benzoic acid, look for tests where their reactions differ: \(\text{FeCl}_3\) test for phenol (positive), \(\text{NaHCO}_3\) for benzoic acid (positive), and \(\text{NaOH}\) test, which shows dissolution for both but does not distinguish them.

Step 5: Determine Correct Answer Based on Distinguishing Reactions

Based on the distinct reactions: Phenol tests positive with \(\text{FeCl}_3\) and can be detected using the Libermann nitroso test, while benzoic acid reacts with \(\text{NaHCO}_3\). We compare options and choose combinations that include these distinguishing reactions.

Key Concepts

Libermann Nitroso Test

The Libermann nitroso test is particularly significant in identifying phenolic compounds. In this test, phenol reacts with sodium nitrite in an acidic medium to form a nitroso phenol. This compound can further react to form a deep green or blue colored complex once the solution is treated with an alkaline solution.
The reaction proceeds through several interesting steps.

• First, an acidic solution of phenol and sodium nitrite generates nitroso phenol.
• When alkali is added, the complex turns green or blue, confirming the presence of phenolic groups.

It is important to note that this specific color change reaction is characteristic for phenolic compounds such as phenol, but not for benzoic acid. Hence, the Libermann nitroso test is an effective tool in distinguishing phenol from benzoic acid.

Ferric Chloride Test

The ferric chloride test is a classic chemical test that identifies phenolic hydroxyl groups in phenols. When you add a few drops of ferric chloride ( FeCl_3) to a solution of phenol, a complex forms. This results in a color transition that is typically violet or purple.
Here’s how the test works:

• Phenol acts as a ligand to the ferric ion, forming a phenol-ferric complex.
• This complex is responsible for the distinctive color change visible in the test.

The intensity of the color can help in the semi-quantitative estimation of phenolic content. Benzoic acid, however, does not react with ferric chloride in this manner, thus showing no color change, allowing for clear distinction between phenol and benzoic acid.

Chemical Reaction with NaHCO\(_3\)

The reaction with sodium bicarbonate (NaHCO_3) serves as a straightforward method to verify the presence of carboxylic acids in compounds like benzoic acid. When benzoic acid is treated with NaHCO_3, it reacts to release carbon dioxide gas. This reaction is evident through the effervescence (bubbling) observed.

• The chemical equation for this reaction is:\[ \text{C}_6\text{H}_5\text{COOH} + \text{NaHCO}_3 \rightarrow \text{C}_6\text{H}_5\text{COONa} + \text{H}_2\text{O} + \text{CO}_2 \uparrow \]

Phenol, however, being less acidic, does not react with NaHCO_3 to produce carbon dioxide. This reaction distinguishes benzoic acid from phenol clearly, with effervescence indicating the presence of an acid strong enough to decompose bicarbonate.