The compound shown has $p K_{a}$ of about 4.1, and the first $p K_{a}$ of carbonic acid is about 6.4. What happens when it is added to aqueous sodium hydrogencarbonate?

ANothing happens, because all phenols are weaker acids than carbonic acid.
BCarbon dioxide is evolved, because the compound is a stronger acid than carbonic acid.
CThe compound is reduced to the amine.
DThe compound is converted into a nitrated ether.

Answer: B

The compound is 2,4-dinitrophenol. Two nitro groups on the ring make it much more acidic than ordinary phenols, and its

p K_{a}

of 4.1 is lower than that of

H X_{2} CO X_{3}

. It therefore protonates the hydrogencarbonate ion and releases

CO X_{2}

, which phenol itself cannot do.