Read the passage carefully and answer the Questions.
Carboxylic acids are the earliest organic compounds isolated from nature. These are also known as organic acids but are weaker acids as compared to mineral acids. However, they are more acidic than alcohols and phenols because of greater resonance stabilization of the parent acid and its conjugate base. The acidic strength is determined by the Ka, which is called the acidity constant. The greater the value of Ka, the greater the tendency of the acid to ionize and hence the stronger the acid. The acidic strength of the saturated aliphatic carboxylic acid depends upon the inductive effect and its position with respect to - COOH groups. Electron-donating substituents tend to decrease whereas electron -withdrawing substituents tend to increase the acidic strength. The acidic strength of the aromatic carboxylic acid, on the other hand depends upon both the inductive effect and the resonance effect of the substituents.
\( pK_a \) of chloroacetic acid is lower than acetic acid because of
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → due to -I effect of Cl atom and +I effect of methyl group.
The acid strength of a saturated aliphatic acid depends on the inductive effect of the group attached to the –COOH. An electron-withdrawing group (−I) draws electron density away from the carboxylate ion, spreads its negative charge and stabilises it, so the acid becomes stronger. An electron-donating group (+I) pushes electron density towards the carboxylate ion and destabilises it, so the acid becomes weaker.
\( \displaystyle \ce{ClCH2COOH + H2O <=> ClCH2COO- + H3O+} \)
\( \displaystyle \ce{CH3COOH + H2O <=> CH3COO- + H3O+} \)
In chloroacetic acid the electronegative chlorine atom withdraws electrons (−I) and stabilises the chloroacetate ion. In acetic acid the methyl group donates electrons (+I) and destabilises the acetate ion. Chloroacetic acid therefore ionises more, has the larger Ka and the lower pKa (about 2.9, against 4.76 for acetic acid).
Why the other options are incorrect
Option (1) mesomeric effect — Incorrect. The mesomeric effect works through conjugated pi bonds; the chlorine is on an sp3 \( \ce{CH2} \) carbon, so it is not conjugated with the carboxylate group.
Option (2) Resonance effect — Incorrect. This is another name for the mesomeric effect; resonance stabilises the carboxylate group in both acids alike and does not explain the difference.
Option (4) Due to Hyperconjugation — Incorrect. Hyperconjugation involves C–H bonds next to a pi system or a carbocation; it is not what makes the chlorine-substituted acid stronger.
NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids – …
