Read the passage given and answer the question.

Phenol and alcohol have the same functional group (-OH) but phenols are stronger acids than alcohols due to the resonance stabilization of phenoxide ion than alkoxide ion \((-RO^-)\). Electron withdrawing groups tend to increase the acidic strength while electron releasing groups tend to decrease it.

Which of the following reagent is used to distinguish between phenol and benzoic acid?

Neutral \(FeCl_3\)

The correct answer is option 2. Neutral \(FeCl_3\).

Let us go through the detailed explanation of how to distinguish between phenol and benzoic acid using the reagents listed, focusing on why neutral \(FeCl_3\) is the correct choice.

Phenol and Benzoic Acid: Basic Properties

Phenol: Phenol is an aromatic compound with a hydroxyl group (-OH) directly attached to the benzene ring.

Benzoic Acid: Benzoic acid is an aromatic carboxylic acid with a carboxyl group (-COOH) attached to the benzene ring.

Reactions with Neutral \(FeCl_3\) (Ferric Chloride)

Phenol reacts with neutral ferric chloride (\(FeCl_3\)) to form a colored complex. This is because phenol can act as a ligand, coordinating with the ferric ion (\(Fe^{3+}\)) to form a ferric phenoxide complex. This complex typically exhibits a violet or purple color.

The violet or purple color indicates the presence of phenol.

Benzoic acid does not react in the same way with neutral ferric chloride. The carboxyl group (-COOH) of benzoic acid does not form a colored complex with \(FeCl_3\). Therefore, when benzoic acid is treated with neutral ferric chloride, no color change is observed.

Why Neutral \(FeCl_3\) is Effective

The formation of a colored complex with neutral \(FeCl_3\) is a characteristic test for phenols, not carboxylic acids like benzoic acid. This makes neutral \(FeCl_3\) an excellent reagent for distinguishing between phenol and benzoic acid because phenol will give a clear positive test (violet color), while benzoic acid will not.

Other Reagents:

Tollen's reagent:

Used to distinguish between aldehydes and ketones. It gives a silver mirror with aldehydes, but this test does not apply to phenol or benzoic acid.

Molisch reagent:

Used to detect carbohydrates. Phenol and benzoic acid do not give a positive result with Molisch reagent, so it cannot distinguish between them.

Aqueous \(NaOH\):

Both phenol and benzoic acid can react with aqueous \(NaOH\) to form water-soluble sodium salts:

Since both compounds react similarly with \(NaOH\), this reagent cannot distinguish between them.

Conclusion

Neutral \(FeCl_3\) is the reagent that effectively distinguishes between phenol and benzoic acid:

Phenol: Reacts with neutral \(FeCl_3\) to give a violet or purple complex.

Benzoic Acid: Does not react with neutral \(FeCl_3\) to give any color change.

Thus, the correct answer is option 2: Neutral \(FeCl_3\).