JEE Main 2025 Apr 7 Shift 1, Chemistry Q15: Inductive Effect

JEE Main2025Apr 7, Shift 1Chemistry

Which of the following compounds is least likely to give effervescence of CO $_{2}$ in presence of aq. NaHCO $_{3}$ ?

Solution

Concept: Aqueous NaHCO $_{3}$ liberates CO $_{2}$ only from acids stronger than carbonic acid ( $p K_{a} \approx 6.37$ ). So the question reduces to identifying which compound has $p K_{a} > 6.37$ (i.e. weaker than H $_{2}$ CO $_{3}$ ).

Option-by-option:

(1) Picric acid: Three strongly electron-withdrawing NO $_{2}$ groups (two ortho, one para) stabilise the phenoxide enormously. $p K_{a} \approx 0.4$ . Far more acidic than H $_{2}$ CO $_{3} \Rightarrow$ releases CO $_{2}$ .

(2) 4-Nitrobenzoic acid: A carboxylic acid, already more acidic than H $_{2}$ CO $_{3}$ ( $p K_{a} \approx 3.4$ ), and further strengthened by the para-NO $_{2}$ group $\Rightarrow$ releases CO $_{2}$ .

(3) Anilinium chloride: The conjugate acid of aniline has $p K_{a} \approx 4.6$ . More acidic than H $_{2}$ CO $_{3} \Rightarrow$ releases CO $_{2}$ .

(4) 3-Nitrophenol: The NO $_{2}$ group at the meta position cannot stabilise the phenoxide by resonance (only weakly by induction). The $p K_{a}$ is about 8.4, which is weaker than carbonic acid. So no CO $_{2}$ is liberated.

$∴$ The compound least likely to give CO $_{2}$ with aq. NaHCO $_{3}$ is 3-nitrophenol $\Rightarrow$ Option (4).

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JEE Main 2025 Apr 7 Shift 1, Chemistry Q15: Inductive Effect

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