Question

Compound(s) which will liberate carbon dioxide with sodium bicarbonate ($NaHCO_3$) solution is/are : A = Urea, B = Benzoic acid, C = p-Nitrophenol

• A. A and B only
• B. C only
• C. B and C only
• D. B only

Hint:

The $NaHCO_3$ test is used to distinguish carboxylic acids from phenols. Only acids with $pK_a<6.4$ (approx) can displace $CO_2$.

Answer 1

The Correct Option is D

Step 1: A compound liberates $CO_2$ from $NaHCO_3$ if it is a stronger acid than carbonic acid ($H_2CO_3$).
Step 2: Benzoic acid (Carboxylic acids) are stronger than $H_2CO_3$ and give a positive test.
Step 3: Phenols (including p-nitrophenol) are generally weaker than $H_2CO_3$ and do not react with bicarbonate (Exception: 2,4,6-trinitrophenol or Picric acid).
Step 4: Urea is basic/neutral and does not react.