JEE Challenger
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Identification of Compound Failing to Liberate Carbon Dioxide with Sodium Bicarbonate

Identify which of the following compounds will NOT liberate CO2\text{CO}_2 gas upon reaction with an aqueous solution of sodium bicarbonate:

Options

A

Benzoic acid

B

Benzenesulphonic acid

C

Salicylic acid

D

Carbolic acid (Phenol)

Correct

Step-by-Step Solution

To determine which compound does not liberate CO2\text{CO}_2 upon reaction with an aqueous solution of sodium bicarbonate (NaHCO3\text{NaHCO}_3), we consider the acid-base equilibrium involved:

HA+NaHCO3⇌NaA+H2CO3⇌NaA+H2O+CO2↑\text{HA} + \text{NaHCO}_3 \rightleftharpoons \text{NaA} + \text{H}_2\text{CO}_3 \rightleftharpoons \text{NaA} + \text{H}_2\text{O} + \text{CO}_2\uparrow

An acid (HA\text{HA}) can decompose sodium bicarbonate to liberate carbon dioxide (CO2\text{CO}_2) only if it is a stronger acid than carbonic acid (H2CO3\text{H}_2\text{CO}_3), which has a pKa\text{p}K_a of approximately 6.356.35. In other words, the acid must have a pKa<6.35\text{p}K_a < 6.35.

Let us examine the pKa\text{p}K_a values of the given compounds:

  1. Benzenesulphonic acid (C6H5SO3H\text{C}_6\text{H}_5\text{SO}_3\text{H}): It is a strong mineral-like acid with pKa≈−2.8\text{p}K_a \approx -2.8. Since pKa<6.35\text{p}K_a < 6.35, it readily reacts with NaHCO3\text{NaHCO}_3 to liberate CO2\text{CO}_2.
  2. Salicylic acid (2-hydroxybenzoic acid2\text{-hydroxybenzoic acid}): The ortho-hydroxy group stabilizes the conjugate base through intramolecular hydrogen bonding (the ortho-effect), giving it a pKa≈2.98\text{p}K_a \approx 2.98. Since pKa<6.35\text{p}K_a < 6.35, it liberates CO2\text{CO}_2.
  3. Benzoic acid (C6H5COOH\text{C}_6\text{H}_5\text{COOH}): A carboxylic acid with pKa≈4.20\text{p}K_a \approx 4.20. Since pKa<6.35\text{p}K_a < 6.35, it decomposes NaHCO3\text{NaHCO}_3 to liberate CO2\text{CO}_2.
  4. Carbolic acid (Phenol, C6H5OH\text{C}_6\text{H}_5\text{OH}): A very weak acid with pKa≈9.95\text{p}K_a \approx 9.95. Because its pKa\text{p}K_a is substantially greater than that of carbonic acid (pKa≈6.35\text{p}K_a \approx 6.35), phenol is a weaker acid than H2CO3\text{H}_2\text{CO}_3. The acid-base equilibrium lies heavily to the left, meaning it fails to decompose NaHCO3\text{NaHCO}_3 and therefore does NOT liberate CO2\text{CO}_2 gas.

Thus, carbolic acid (phenol) does not liberate CO2\text{CO}_2.

Correct Option: (D)