Question

Give simple chemical tests to distinguish between the following pairs of compounds. 

1. Propanal and Propanone 

2. Acetophenone and Benzophenone 

3. Phenol and Benzoic acid 

4. Benzoic acid and Ethyl benzoate 

Answer 1

1. Propanal is an aldehyde and propanone is a ketone. These can be distinguished by 

(a) Silver mirror test: Propanal reduces Tollens reagent to silver which propanone does not.

(b) Iodoform test: Propanone will give iodoform test while propanal will not give this test. Propanone gives yellow ppt of iodoform

2. Acetophenone and Benzophenone: 

Acetophenone contains CH₃CO group, as such it will give iodoform test with NaOI (NaOH⁺I₃) while no such group is there in benzophenone, so such it will not give iodoform test.

3. Phenol and Benzoic acid

(a) Sodium bicarbonate test: Benzoic acid is a stronger acid than carbonic acid (H₂O + CO₂), as such it will decompose sodium bicarbonate to give CO₂. On the other hand, phenol is weaker acid than carbonic acid, as a result it will not decompose sodium bicarbonate.
C₆H₅COOH + NaHCO₃ → C₆H₅COONa + CO₂ + H₂O

C₆H₅OH + NaHCO₃ → No action

(b) Ferric chloride test: Phenol gives violet colouration with neutral FeCl₃, solution while benzoic acid gives buff coloured ppt. of ferric benzoate.

4. Benzoic acid and Ethyl benzoate: Benzoic acid is an acid and ethyl benzoate is an ester. These may be distinguished by

(a) Sodium bicarbonate test: Benzoic acid will decompose sodium bicarbonate to give CO₂ while ethyl benzoate will not.
C₆H₅COOH + NaHCO₃ → C₆H₅COONa + CO₂ + H₂O
C₆H₅COOC₂H₅ + NaHCO₃ → no reaction

(b) Litmus test: Benzoic acid term blue litmus to red, while ethyl benzoate will not.