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

Short answer

(b) 1, 2 and 4

Step-by-step solution

Understanding the Compounds

Phenol is an aromatic compound with a hydroxyl group directly attached to a benzene ring. Benzoic acid is a carboxylic acid with the carboxyl group attached to a benzene ring. Both have distinct chemical reactivities that can be used for differentiation.

Libermann Nitroso Test

This test is typically used for detecting phenols. It involves the formation of a colored complex with nitrous acid. Phenols give a characteristic deep blue or green color, while benzoic acid does not react or produce this color.

Reaction with NaHCO3

Benzoic acid, being an acid, can react with sodium bicarbonate (\(\mathrm{NaHCO}_3\)), producing carbon dioxide gas, evident as bubbles. Phenol, being much less acidic than benzoic acid, will not react with \(\mathrm{NaHCO}_3\) to produce bubbles.

Reaction with NaOH

Both phenol and benzoic acid can react with sodium hydroxide (\(\mathrm{NaOH}\)). However, the extent of reaction is different. Benzoic acid reacts easily forming sodium benzoate, a soluble salt. Phenol reacts too, forming sodium phenoxide, but with less effervescence compared to an acid-bicarbonate reaction.

Ferric Chloride Test

Phenols react with ferric chloride to form a colored complex, usually purple or blue. Benzoic acid does not give a characteristic color when mixed with ferric chloride solution, helping to distinguish it from phenol.

Combining Tests for Distinguishing

Based on the above tests, we can distinguish the compounds: 1. Libermann nitroso test distinguishes phenol (positive result) from benzoic acid (negative result). 2. Reaction with \(\mathrm{NaHCO}_3\) distinguishes benzoic acid (bubbles) from phenol (no reaction). 4. Ferric chloride test distinguishes phenol (colored complex) from benzoic acid (no color).

Key concepts

Phenol

Phenol is an organic compound characterized by the presence of a hydroxyl group \((-OH)\) attached directly to an aromatic benzene ring. This structure gives phenol unique physical and chemical properties. It is slightly acidic, with the ability to lose a hydrogen ion \((H^+)\), though much less acidic compared to carboxylic acids like benzoic acid. Phenols are known for their distinct smell and are widely used in industrial and chemical applications.

The acidity of phenol is attributed to the resonance stabilization provided by the aromatic ring, which allows the phenoxide ion (the conjugate base of phenol) to have a stable electron arrangement. However, the acid character is weak, which is an essential factor in differentiating phenol from more robust acids, such as benzoic acid, in chemical tests.

Benzoic Acid

Benzoic acid is a simple aromatic carboxylic acid with the formula \(C_6H_5COOH\). It consists of a benzene ring attached to a carboxyl group \((-COOH)\). This structure makes benzoic acid a strong organic acid compared to phenol, allowing it to readily donate a proton \((H^+)\) in reactions. This acidity is attributed to the resonance stabilization of the carboxylate ion, which forms upon the ionization of the carboxyl group.

Benzoic acid is commonly used as a food preservative due to its antifungal properties. It is also significant in chemical synthesis and industrial applications, highlighting the importance of its identification and differentiation from other aromatic compounds such as phenol.

Chemical Tests for Phenols

Several chemical tests are available to specifically identify phenols, which exploit their unique reaction characteristics:

• Libermann Nitroso Test: This test involves the reaction of phenol with nitrous acid, resulting in a deep blue or green complex indicative of phenols. Benzoic acid, on the other hand, does not produce this coloration.
• Ferric Chloride Test: Phenols react with ferric chloride to form a colored complex, typically purple or blue, depending on the specific structure of the phenol. This property distinguishes phenols from compounds like benzoic acid, which do not react to form such colored complexes.

These tests are critical in laboratories for differentiating phenols from other compounds, enabling proper identification in qualitative analysis.

Carboxylic Acids Identification

Carboxylic acids, like benzoic acid, can be identified through specific chemical tests that focus on their acidity and reactivity:

• Reaction with Sodium Bicarbonate \((\mathrm{NaHCO}_3)\): Carboxylic acids like benzoic acid react with \(\mathrm{NaHCO}_3\) to produce carbon dioxide gas, visible as bubbles. This reaction clearly distinguishes them from less acidic compounds like phenol, which do not react in this manner.
• Reaction with Sodium Hydroxide \((\mathrm{NaOH})\): Carboxylic acids readily react with strong bases like \(\mathrm{NaOH}\) to form soluble salts, such as sodium benzoate. Although phenol also reacts with \(\mathrm{NaOH}\) to form sodium phenoxide, its reaction is not as pronounced as that of carboxylic acids.

These tests are essential in confirming the presence of carboxylic acids, providing clear evidence of their unique chemical properties.