Distinguishing Between Phenol and Benzoic Acid

Phenol (C₆H₅OH) and benzoic acid (C₆H₅COOH) are both aromatic compounds but differ in functional groups: phenol has a hydroxyl group attached directly to the benzene ring, while benzoic acid has a carboxylic acid group.

Step 1: Understand the Chemical Behavior

Phenol is weakly acidic due to resonance stabilization of the phenoxide ion, but less acidic than carboxylic acids. Benzoic acid is a stronger acid due to the -I effect of the carbonyl group.

Step 2: Evaluate Each Reagent

Tollen's reagent: Used to detect aldehydes; neither phenol nor benzoic acid are aldehydes, so no reaction with either. Not useful for distinction.

Aqueous NaOH: Both react: phenol forms sodium phenoxide (soluble), benzoic acid forms sodium benzoate (soluble). No distinct observation. Not useful.

Neutral FeCl₃: Phenol gives a violet color due to complex formation; benzoic acid gives a buff-colored precipitate of ferric benzoate. This shows a clear visible difference.

Molisch reagent: Used for carbohydrates; neither compound is a carbohydrate, so no reaction. Not useful.

Step 3: Conclusion

Only neutral FeCl₃ provides a distinct test: violet color for phenol, buff precipitate for benzoic acid.

Final Answer

Neutral FeCl₃

Related Topics

• Acidity of organic compounds
• Functional group tests
• Aromatic compound reactions

Key Formulae and Theory

Phenol with FeCl₃: $6C{H}_{5}OH+\mathrm{Fe}{\mathrm{Cl}}_3\to \left[\mathrm{Fe}\right(O{C}_6{H}_5{)}_6{]}^3+3H\mathrm{Cl}$ (violet complex)

Benzoic acid with FeCl₃: $3C{H}_{6}H{O}_2+\mathrm{Fe}{\mathrm{Cl}}_3\to (C{H}_{6}H{O}_2{)}_3\mathrm{Fe}+3H\mathrm{Cl}$ (buff precipitate)

Acidity order: Benzoic acid (pK_a ≈ 4.2) > Phenol (pK_a ≈ 10).