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Q.Assertion (A): The pKapK_a of ethanoic acid is lower than that of Cl−CH2−COOHCl-CH_2-COOH.
Reason (R): Chlorine shows electron withdrawing (−I-I) effect which increases the acidic character of Cl−CH2−COOHCl-CH_2-COOH.
[Codes (A)-(D) as in the Assertion-Reason instruction.]

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The key idea is that the −I-I effect of chlorine stabilises the conjugate base of chloroacetic acid, making it a stronger acid (lower pKapK_a) than ethanoic acid. Both Assertion and Reason are true, and Reason correctly explains Assertion.

Let’s understand why this happens. Acidity is all about the stability of the conjugate base after the acid loses a proton. For carboxylic acids, the conjugate base is a carboxylate anion (RCOO−RCOO^-). The more stable this anion, the stronger the acid — and the lower its pKapK_a.

Ethanoic acid (CH3COOHCH_3COOH) has a methyl group attached to the carboxyl carbon. A methyl group is weakly electron-donating via the inductive effect (+I+I), which slightly destabilises the negative charge on the carboxylate ion. That makes ethanoic acid a moderately weak acid, with pKa≈4.76pK_a \approx 4.76.

Now replace one hydrogen of the methyl group with chlorine. Chlorine is highly electronegative — it pulls electron density towards itself through the sigma bonds. This is the electron-withdrawing inductive effect (−I-I). In Cl−CH2−COOHCl-CH_2-COOH, the chlorine pulls electrons away from the carboxyl group, which delocalises and stabilises the negative charge on the conjugate base. A more stable conjugate base means a stronger acid.

  1. Compare the conjugate bases.

    For ethanoic acid: CH3COO−CH_3COO^- — the methyl group pushes electrons, making the negative charge less stable.

    For chloroacetic acid: Cl−CH2−COO−Cl-CH_2-COO^- — the chlorine pulls electrons, spreading out the negative charge and making the anion more stable.

  2. Relate stability to pKapK_a.

    pKa=−log⁡KapK_a = -\log K_a. A stronger acid has a larger KaK_a and therefore a smaller pKapK_a. Since chloroacetic acid is the stronger acid, its pKapK_a is lower.

    pKa(ethanoic)≈4.76;pKa(chloroacetic)≈2.87pK_a(\text{ethanoic}) \approx 4.76 \quad ; \quad pK_a(\text{chloroacetic}) \approx 2.87

  3. Check the Reason. …

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