Question: Which one of the following has lowest pK_a value?

(a) CH₃COOH, (b) O₂NCH₂COOH, (c) ClCH₂COOH, (d) HCOOH

Step 1 (Governing Principle): [0.25 Mark]

The acidity of carboxylic acids is determined by the stability of the conjugate base (carboxylate ion). The relationship between acidity and pK_a is inverse:

Electron-Withdrawing Groups (EWG) stabilize the carboxylate anion through the inductive effect ($-I$ effect), thereby increasing acidity and decreasing the pK_a value.

Step 2 (Step-by-Step Comparative Analysis): [0.5 Mark]

• (a) CH₃COOH: Contains a methyl group ($+I$ effect), which destabilizes the carboxylate ion.
• (d) HCOOH: Reference acid with no alkyl group.
• (c) ClCH₂COOH: Contains a Chlorine atom ($-I$ effect), which increases acidity compared to (a) and (d).
• (b) O₂NCH₂COOH: Contains a Nitro group ($-NO_2$). The $-NO_2$ group exerts a much stronger electron-withdrawing ($-I$) effect than the Chlorine atom.

Comparing the strength of the $-I$ effect: $$-NO_2 > -Cl > -H > -CH_3$$ Therefore, the order of acidity is: $$\text{O}_2\text{NCH}_2\text{COOH} > \text{ClCH}_2\text{COOH} > \text{HCOOH} > \text{CH}_3\text{COOH}$$

Step 3 (Final Concluded Answer): [0.25 Mark]

Since the pK_a value is inversely proportional to acid strength, the compound with the strongest electron-withdrawing group will have the lowest pK_a value.

Correct Option: (b) O₂N - CH₂ - COOH

💡 EL10 Concept Insight: Think of the carboxylate ion as a "hot potato" (negative charge) that needs to be cooled down; the $-NO_2$ group acts like a powerful vacuum cleaner that sucks away that extra charge, making the acid much stronger and its pK_a value much smaller.