The correct order of increasing $pK_a$ values of the compounds:
(A) $CH_3COOH$
(B) $CH_2OCH_2COOH$
(C) $Cl_3COOH$
(D) $(CH_3)_3CCOOH$
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C) < (B) < (A) < (D)
pKa is inversely related to acidity.
Stronger acids have lower pKa values, while weaker acids have higher pKa values.
The acidity of carboxylic acids depends on the electron-withdrawing (−I) or electron-donating (+I) effect of substituents.
(C) Cl₃COOH (Trichloroacetic acid)
Three chlorine atoms exert a very strong −I effect, stabilizing the carboxylate ion strongly.
Therefore, it is the strongest acid → lowest pKa.
(B) CH₂OCH₂COOH
The –O– group shows a −I effect, which withdraws electrons and increases acidity compared to acetic acid.
Thus its pKa is higher than (C) but lower than (A).
(A) CH₃COOH (Acetic acid)
The CH₃ group shows a +I effect, which slightly reduces acidity.
Hence its pKa is higher than (B).
(D) (CH₃)₃CCOOH (tert-butyl carboxylic acid)
The tert-butyl group has a strong +I effect, pushing electron density toward the carboxyl group and destabilizing the conjugate base.
Thus it is the weakest acid → highest pKa.
Therefore, the order of increasing pKa is:
Cl₃COOH < CH₂OCH₂COOH < CH₃COOH < (CH₃)₃CCOOH.