Problem 213 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reagents may be used to distinguish between phenol and benzoic acid? [2011] (a) Tollen's reagent (b) Molisch reagent (c) Neutral \(\mathrm{FeCl}_{3}\) (d) Aqueous \(\mathrm{NaOH}\)

Step-by-Step Solution

Answer

Neutral \(\mathrm{FeCl}_{3}\) can be used to distinguish between phenol and benzoic acid.

Step 1: Understanding the reagents

Before determining which reagent can distinguish between phenol and benzoic acid, we need to understand what each reagent does. Tollen's reagent is used for distinguishing aldehydes; Molisch reagent is used for detecting carbohydrates; Neutral \( \mathrm{FeCl}_{3}\) is used to test for phenolic groups, showing a purple color with phenol; and Aqueous \( \mathrm{NaOH}\) is a common test that can react with acidic substances like benzoic acid.

Step 2: Reaction with Neutral \(\mathrm{FeCl}_{3}\)

Neutral \(\mathrm{FeCl}_{3}\) forms a colored complex with phenols. When phenol is treated with Neutral \(\mathrm{FeCl}_{3}\), it gives a characteristic violet or purple color, which is a confirmation of the phenolic group. Benzoic acid does not react with Neutral \(\mathrm{FeCl}_{3}\) in this way, so this reagent can distinguish phenol from benzoic acid.

Step 3: Reaction with Aqueous \(\mathrm{NaOH}\)

Aqueous \(\mathrm{NaOH}\) will react with both phenol and benzoic acid since they are both acidic. Both will form water-soluble sodium salts, so this reagent cannot effectively distinguish between phenol and benzoic acid.

Step 4: Eliminating incorrect options

Tollen's reagent and Molisch reagent do not react with either phenol or benzoic acid. Tollen's reagent is specific for aldehydes and Molisch reagent for carbohydrates. Therefore, they cannot be used to distinguish between phenol and benzoic acid.

Step 5: Selecting the correct reagent

Among the reagents, Neutral \(\mathrm{FeCl}_{3}\) is the only one that can distinguish between phenol and benzoic acid because it forms a colored complex with phenolic groups, providing a visual confirmation for phenol.

Key Concepts

Phenol

Phenol, also referred to as carbolic acid, is a well-known compound in organic chemistry due to its unique properties and reactions. It consists of a hydroxyl group

• Phenol has the formula: C₆H₅OH.
• It's known for having antiseptic properties.
• In its purest form, phenol is a white crystalline solid that is volatile.

The presence of the hydroxyl group attached to an aromatic ring makes phenol slightly acidic. This contributes to phenol's capability to participate in specific chemical reactions, including its reaction with ferric chloride (FeCl₃) test to form a purple complex.
The acidity of phenol, while not as strong as some other acids, is significant enough to influence its reactivity, but it does not react with Tollen's reagent or Molisch reagent.
This makes phenol an excellent candidate for specific tests that leverage its phenolic nature.

Benzoic Acid

Benzoic acid is another fundamental organic compound, often used as a preservative and serving as a building block for more complex chemicals.

• Benzoic acid has the chemical formula: C₆H₅COOH.
• It is an aromatic carboxylic acid, where the carboxyl group is directly attached to the benzene ring.
• Unlike phenol, benzoic acid is a stronger acid, contributing to its broader range of reactions.

This acid is commonly encountered in plants and is a precursor in the synthesis of more complex molecules.
In a lab setting, benzoic acid is distinguishable from phenol by its inability to form complexes with neutral ferric chloride (FeCl₃).
Aqueous NaOH reacts with both benzoic acid and phenol to form soluble salts, but this reaction doesn't help in distinguishing between the two.

FeCl3 Test

The FeCl₃ test is a crucial analytical technique in chemistry for detecting phenolic groups.

• This involves adding neutral ferric chloride to the compound being tested.
• A distinctive violet or purple coloration indicates the presence of phenol.

The FeCl₃ test is particularly useful because it offers a clear, visual method to confirm phenolic presence, since only phenols and related compounds produce this color change
Upon reacting, phenol forms a complex that changes color, but since benzoic acid lacks a free hydroxyl group, it does not undergo this reaction.
Thus, Neutral FeCl₃ is a targeted way to identify phenol in a mixture of organic compounds.

Chemical Reagents Distinction

Chemical reagents are crucial tools for distinguishing between different organic compounds, such as phenol and benzoic acid.

• Specific reagents react uniquely with certain functional groups.
• Using these reagents helps identify and distinguish substances based on their chemical properties.

For phenol and benzoic acid, understanding the reactivity of reagents is essential.
Neutral FeCl₃ is effective for detecting phenolic groups due to the color change it prompts.
Meanwhile, Tollen's reagent and Molisch reagent target different groups and do not react with phenol or benzoic acid.
This specificity allows chemists to systematically and effectively identify unknown samples by leveraging well-characterized reactions.