Question

Which of the following compounds which give $CO_2 \uparrow$ gas on reaction with aq. $NaHCO_3$?

• A. 2,4-Dinitrophenol
• B. Phenol
• C. 4-Nitrophenol
• D. 2-Nitrophenol

Answer 1

Answer: (A)

2,4-Dinitrophenol

Answer 2

A compound reacts with $NaHCO_3$ to produce $CO_2$ gas if it is a stronger acid than carbonic acid. The $pK_a$ of carbonic acid is approximately 6.37. Among the given options: \n* 2,4-Dinitrophenol: $pK_a \approx 4.10$. Since $4.10 < 6.37$, it reacts with $NaHCO_3$ to produce $CO_2$. \n* Phenol: $pK_a \approx 9.95$. Since $9.95 > 6.37$, it does not react. \n* 4-Nitrophenol: $pK_a \approx 7.15$. Since $7.15 > 6.37$, it does not react. \n* 2-Nitrophenol: $pK_a \approx 7.23$. Since $7.23 > 6.37$, it does not react. \nTherefore, only 2,4-dinitrophenol is acidic enough to react with $NaHCO_3$ and evolve $CO_2$ gas.