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 (release of CO₂ gas) occurs when a compound reacts with sodium bicarbonate (NaHCO₃) if the compound is acidic enough to protonate the bicarbonate ion (HCO₃^-) to form carbonic acid (H₂CO₃). Carbonic acid is unstable and decomposes into water and carbon dioxide: HA + HCO₃^- ⇌ A^- + H₂CO₃ H₂CO₃ ⇌ H₂O + CO₂ ↑ This reaction proceeds significantly only if the acid HA is stronger than carbonic acid (pKa of H₂CO₃ ≈ 6.4). We need to identify which compound among the options is sufficiently acidic.

• (A) m-Nitrotoluene: Contains only relatively non-acidic C-H bonds. Not acidic enough.
• (B) Acetone: A ketone. Its α-hydrogens are very weakly acidic (pKa ≈ 19-20). Not acidic enough.
• (C) 2-Naphthol: An alcohol attached to a naphthalene ring (a type of phenol). Phenols are weakly acidic (phenol pKa ≈ 10). While naphthols are slightly more acidic than phenol, they are generally not strong enough to react vigorously with NaHCO₃.
• (D) 2,4,6-Trinitrophenol (Picric Acid): This is a phenol substituted with three powerful electron-withdrawing nitro groups (-NO₂) at the ortho and para positions. These groups strongly stabilize the negative charge on the oxygen in the conjugate base (phenoxide ion) through resonance and inductive effects. This makes picric acid a very strong acid (pKa ≈ 0.4), significantly 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.