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.
Which of the following groups when attached directly does not increase the acidity of the carboxylic acid?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Methyl
A group attached to the –COOH carbon changes the acidity through the stability of the carboxylate ion. A group that withdraws electrons stabilises the ion and increases acidity; a group that donates electrons destabilises it and decreases acidity.
The methyl group is electron-donating (+I). It pushes electron density towards the carboxylate group, so the acetate ion is less stable than the formate ion, and acetic acid (pKa = 4.76) is weaker than formic acid (pKa = 3.75):
\( \displaystyle \ce{CH3COOH} \lt \ce{HCOOH} \quad \text{(acid strength)} \)
Option (1) Vinyl — Increases the acidity. The carbon of –\( \ce{CH=CH2} \) attached to the –COOH group is sp2 hybridised, and an sp2 carbon is more electronegative than an sp3 carbon, so it withdraws electrons and increases acidity.
Option (2) Phenyl — Increases the acidity. For the same reason (an sp2 carbon attached to the carboxyl carbon), direct attachment of phenyl increases the acidity, even though resonance might be expected to decrease it.
Option (3) Methyl — Does not increase the acidity. Methyl is electron-donating (+I); it destabilises the carboxylate ion, so acetic acid is weaker than formic acid.
Option (4) Nitro — Increases the acidity. –\( \ce{NO2} \) is a strong electron-withdrawing group (−I and −R); nitroacetic acid and p-nitrobenzoic acid are much stronger than the unsubstituted acids.
NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids – …
