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?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Neutral FeCl3
Neutral ferric chloride is the reagent for the phenolic group. Phenol forms a coloured iron(III) phenoxide complex and gives a violet colour, while benzoic acid gives a buff (pale brown) precipitate of ferric benzoate. The two results are different, so the reagent tells the two compounds apart.
\( \ce{6C6H5OH + FeCl3 -> [Fe(OC6H5)6]^{3-} + 3H+ + 3HCl} \) (violet)
Why the other options are incorrect
Option (1) Tollen’s reagent — Incorrect. It is reduced only by aldehydes (and a few other reducing compounds); neither phenol nor benzoic acid has an aldehyde group, so both give no silver mirror.
Option (3) Molisch reagent — Incorrect. It is a test for carbohydrates; both compounds give a negative result.
Option (4) Aq. NaOH — Incorrect. Phenol and benzoic acid are both acidic enough to dissolve in aqueous NaOH, so both behave alike.