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? (a) Tollen's reagent (b) Molisch reagent (c) Neutral \(\mathrm{FeCl}_{3}\) (d) Aqueous \(\mathrm{NaOH}\)

Step-by-Step Solution

Answer

Neutral \\( ext{FeCl}_{3}\\) is used to distinguish between phenol and benzoic acid.

Step 1: Understand the properties of phenol and benzoic acid

Phenol is an aromatic compound with a hydroxyl group attached to a benzene ring, while benzoic acid has a carboxylic acid group attached to a benzene ring. Their chemical structures lead to different reactions with certain reagents.

Step 2: Analyze the interaction with Neutral FeCl3

Neutral \( ext{FeCl}_{3}\) reacts with phenolic compounds to give a violet or purple color, because phenols form a complex with iron (III) ions. Benzoic acid doesn't show the same color change with \( ext{FeCl}_{3}\).

Step 3: Examine other reagents

Tollen's reagent is typically used to detect aldehydes, and Molisch reagent is used to test for carbohydrates. Aqueous NaOH can react with both phenol and benzoic acid, forming water-soluble salts, thus does not distinctly differentiate between them.

Step 4: Conclude which reagent can distinguish the two

Given the reactions, Neutral \( ext{FeCl}_{3}\) is the only reagent that clearly distinguishes phenol from benzoic acid due to the color change that occurs with phenol.

Key Concepts

Phenol

Phenol is a fascinating aromatic compound widely used in organic chemistry. It consists of a hydroxyl group \(-OH\) bonded directly to a benzene ring. This special arrangement, referred to as a phenolic group, gives phenol distinct characteristics. Known for its slightly acidic properties, phenol can partially dissolve in water, producing a weak acidic solution. When solutions of phenol are involved in chemical reactions, they can form unique products such as phenoxide ions, especially in the presence of bases.

Some key properties of phenol include:

• White crystalline solid at room temperature
• Higher acidity compared to aliphatic alcohols due to resonance stabilization of the phenoxide ion
• Pungent odor typical of benzene derivatives

These properties make phenol an essential subject of study, especially when it comes to understanding its interactions with various reagents.

Benzoic Acid

Benzoic acid is another aromatic compound where a carboxylic acid group \(-COOH\) is directly attached to a benzene ring. This configuration gives benzoic acid both acidic and aromatic properties. It is most well-known for its role as a preservative in the food industry, as it inhibits the growth of mold, yeast, and some bacteria.

Important characteristics of benzoic acid include:

• White or colorless crystalline appearance
• Higher melting point compared to phenol
• Presence of the carboxyl group makes it significantly more acidic than phenol

When benzoic acid reacts with bases, it forms benzoate salts, which are water-soluble. These properties influence how benzoic acid interacts with different chemical reagents, setting it apart from phenol in various tests.

Neutral FeCl3

Neutral FeClm{3}, or iron(III) chloride in a neutral solution, serves as a fantastic reagent for identifying phenolic compounds. This is because phenols react distinctively with FeClm{3}, forming a colorful complex due to the presence of the hydroxyl group in phenols successfully coordinating with iron(III) ions.

The color change observed, typically purple or violet, serves as a clear indicator and can be used to distinguish phenol from other similar aromatic compounds like benzoic acid, which doesn't produce any color change with FeClm{3}. This reaction is often utilized in laboratories to help quickly identify phenolic substances.

Chemical Reagents

Chemical reagents are substances or compounds added to a system to cause a chemical reaction or test if a reaction occurs. They play critical roles in the vast world of chemistry and are especially vital in distinguishing between substances with similar compositions, like phenol and benzoic acid.

For distinguishing these two compounds, the following reagents come into consideration:

• Tollen's reagent: It is commonly used for detecting aldehydes, not effective for phenol or benzoic acid.
• Molisch reagent: Utilized to test for carbohydrates, again not suitable for distinguishing phenolics.
• Neutral FeClm{3}: This reagent, as discussed, effectively distinguishes phenol by inducing a color change, unlike with benzoic acid.
• Aqueous NaOH: Although it reacts with both phenol and benzoic acid, converting them into their respective salts, it doesn’t help differentiate the two.

Understanding how these reagents function and their specific applications is essential for accurately identifying different chemical substances in analytical chemistry.