Question

Compound given below will produce effervescence when mixed with aqueous sodium bicarbonate solution

• A.
• B.
• C.
• D.

Hint:

Using a simple frame or just bolding for the box
Key Points:
Reaction with NaHCO$_3$ (producing CO$_2$ effervescence) is a test for acids stronger than carbonic acid (pKa $\approx$ 6.4).
Carboxylic acids (RCOOH) typically react.
Simple phenols (like phenol, naphthol) generally do not react or react very weakly.
Phenols substituted with strong electron-withdrawing groups (like -NO$_2$ at o/p positions) can become strongly acidic (e.g., picric acid) and will react.
Alcohols, ketones, and simple hydrocarbons are not acidic enough.

Answer 1

The Correct Option is D

Effervescence (CO₂ gas release) happens when an acidic compound reacts with sodium bicarbonate (NaHCO₃). This occurs if the compound is acidic enough to donate a proton to the bicarbonate ion (HCO₃^-), forming unstable carbonic acid (H₂CO₃), which then breaks down into water and carbon dioxide: HA + HCO₃^- ⇌ A^- + H₂CO₃ H₂CO₃ ⇌ H₂O + CO₂ ↑ The reaction is significant only if the acid HA is stronger than carbonic acid (pKa ≈ 6.4). We need to determine which compound among the choices is sufficiently acidic.

• (A) m-Nitrotoluene: Contains non-acidic C-H bonds; not acidic enough.
• (B) Acetone: A ketone; its α-hydrogens are weakly acidic (pKa ≈ 19-20); not acidic enough.
• (C) 2-Naphthol: A phenol; phenols are weakly acidic (phenol pKa ≈ 10); generally not strong enough to react vigorously with NaHCO₃.
• (D) 2,4,6-Trinitrophenol (Picric Acid): This phenol has three electron-withdrawing nitro groups (-NO₂). These groups stabilize the negative charge on the conjugate base, making picric acid a strong acid (pKa ≈ 0.4), much stronger than carbonic acid.

Therefore, picric acid (D) is the only compound listed that is strong enough to react with aqueous sodium bicarbonate and produce effervescence.