Question

Give chemical tests to distinguish between the following pairs of compounds:
(i) Phenol and Benzoic acid:
(ii) Propanal and Propanone:

Hint:

For distinguishing phenol and benzoic acid, FeCl$_3$ is used to form a violet color with phenol. For distinguishing aldehydes from ketones, Tollens' and Fehling's reagents are useful.

Answer 1

(i) Phenol and Benzoic acid:
- To distinguish phenol and benzoic acid, add neutral FeCl$_3$.
- Phenol: When neutral FeCl$_3$ is added to phenol, a violet color complex is formed due to the coordination of the Fe$^{3+}$ ion with the oxygen of the phenol's hydroxyl group.
- Benzoic acid: Benzoic acid, which is a carboxylic acid, does not form a complex with FeCl$_3$, hence no color change is observed.
- Alternatively, you can add bromine water to each compound:
- Phenol: Phenol reacts with bromine water to form 2,4,6-tribromophenol, a white precipitate.
- Benzoic acid: Benzoic acid does not react with bromine water.
(ii) Propanal and Propanone:
- Tollens' reagent: This reagent is a mild oxidizer used to distinguish aldehydes from ketones. Propanal, being an aldehyde, will reduce Tollens' reagent to silver metal, which forms a silver mirror.
- Propanone: Propanone, a ketone, does not react with Tollens' reagent. Hence, no silver mirror will form.
- Fehling’s solution: This is another test to distinguish between aldehydes and ketones.
- Propanal: It will reduce Fehling’s solution, which contains Cu$^{2+}$ ions, to a red precipitate of Cu$_2$O.
- Propanone: Being a ketone, it will not reduce Fehling’s solution, and no precipitate will form.