Relative acidic strength of Carboxylic acids and their derivatives
Carboxylic acids are weak acids. Mineral acids like HCl or HNO3 are very strong acids and non-edible, but many carboxylic acids are edible acids. Carboxylic acids are more acidic than phenol. The conjugate base of carboxylic acid is stabilized by two equivalent resonating structures in which the negative charge is at the more electronegative oxygen atom. Electron withdrawing group (EWG) increases the acidity of carboxylic acids while electron donating group (EDG) decreases the acidic strength of carboxylic acids. The acidic nature of carboxylic acid is also affected by direct attachment of groups such as phenyl or vinyl to the carboxylic acid. They increase the acidity of corresponding carboxylic acids.
It is due to the greater electronegativity of sp2 hybridized carbon to which carboxylic carbon is attached. This acidic character of carboxylic acid is based on its pKa values. The smaller the pka values, the stronger the acid. The effect of the following groups in increasing acidic order is
Ph < I < Br < Cl < F < CN < NO2 < CF3
Among the following acids, which acid is the strongest acid ?
CF3COOH, CCl3COOH, CH3COOH, HCOOH
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → CF3 COOH
An electron withdrawing group attached to the carboxyl group pulls electron density away from the carboxylate ion and stabilises it, so the acid becomes stronger. The strength of the −I effect of the halogens is F > Cl > Br > I, and more halogen atoms give a larger effect. An electron donating group such as –\( \ce{CH3} \) has the opposite effect and weakens the acid.
Fluorine is the most electronegative element, so –\( \ce{CF3} \) is the strongest electron withdrawing group here. The pKa values confirm the order (smaller pKa means a stronger acid):
\( \displaystyle \mathrm{CF_3COOH}\ (0.23) > \mathrm{CCl_3COOH}\ (0.64) > \mathrm{HCOOH}\ (3.75) > \mathrm{CH_3COOH}\ (4.76) \)
So the strongest acid is CF3COOH.
Why the other options are incorrect
Option (1) CCl3 COOH — Incorrect. \( \ce{CCl3COOH} \) is also a strong acid, but chlorine is less electronegative than fluorine, so its electron withdrawing effect is weaker.
Option (2) CH3 COOH — Incorrect. The –\( \ce{CH3} \) group is electron donating, so \( \ce{CH3COOH} \) is the weakest acid of the four.
Option (3) HCOOH — Incorrect. HCOOH has no electron withdrawing group; it is stronger than acetic acid but far weaker than the halogenated acids.
NCERT (2026-27 edition):
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