Question

Correct order of pKa values of following acids is:

• A. B $>$ D $>$ E $>$ A $>$ C
• B. C $>$ A $>$ E $>$ D $>$ B
• C. B $>$ A $>$ D $>$ E $>$ C
• D. E $>$ C $>$ D $>$ A $>$ B

Hint:

Higher acidity = Lower pKa. More "Cl" atoms or EWGs make the proton easier to lose!

Answer 1

The Correct Option is C

Step 1: Concept pKa is inversely proportional to acid strength. A stronger acid has a lower pKa.

Step 2: Meaning Acid strength is increased by electron-withdrawing groups (EWG) and decreased by electron-donating groups (EDG).

Step 3: Analysis $CCl_{3}COOH$ (C) is the strongest due to three highly electronegative Cl atoms (-I effect). $CH_{3}COOH$ (B) is the weakest due to the $+I$ effect of the methyl group. HCOOH (A) is stronger than acetic acid. Benzoic acid (D) and $p$-chlorobenzoic acid (E) fall in between, with E being stronger than D due to the Cl substituent.

Step 4: Conclusion The acidity order is C $>$ E $>$ D $>$ A $>$ B. Therefore, the pKa order (weakest acid to strongest) is B $>$ A $>$ D $>$ E $>$ C. Final Answer: (C)