Carboxylic Acids: Types of Reactions
Carboxylic acid exhibits a number of reactions depending on the involvement of a specific part of the -COOH group. It mainly includes nucleophilic acyl substitution reaction, in which a hydroxy group of -COOH is replaced by another nucleophile, and the acidic behavior of carboxylic acids, which is due to the cleavage of O-H bond in -COOH group. The former reaction leads to the conversion of carboxylic acid into a variety of different derivatives/categories of organic compounds, whereas the latter converts it into salts which find applications in many industries, like soap and detergent synthesis and pharmaceutical industries. Other important types of chemical reactions involve reduction of -COOH group, decarboxylation, electrolysis and substitution reactions in the hydrocarbon part at α-carbon atom.
Which of the following carboxylic acid has the highest pKa value?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 4-methoxybenzoic acid
pKa = −log Ka, so a higher pKa means a weaker acid. The strength of a substituted benzoic acid depends on how the substituent affects the carboxylate ion formed on losing \( \ce{H^{+}} \):
Electron withdrawing groups (–\( \ce{NO2} \), –CN) spread out the negative charge and stabilise the carboxylate ion. The acid becomes stronger and its pKa lower.
Electron donating groups (–\( \ce{OCH3} \), which donates by resonance from the para position) push more electron density onto the carboxylate ion and destabilise it. The acid becomes weaker and its pKa higher.
The pKa values of the four acids are:
\( \displaystyle \text{4-nitrobenzoic acid }(3.41) < \text{4-cyanobenzoic acid }(3.55) \)
\( \displaystyle < \text{benzoic acid }(4.19) < \text{4-methoxybenzoic acid }(4.46) \)
So the acid with the highest pKa, the weakest of the four, is 4-methoxybenzoic acid.
Why the other options are incorrect
Option (1) benzoic acid — Incorrect. It has no electron donating group, so it is stronger than 4-methoxybenzoic acid and has a lower pKa (4.19).
Option (3) 4-cyanobenzoic acid — Incorrect. –CN withdraws electrons, which increases acidity and lowers pKa (3.55).
Option (4) 4-nitrobenzoic acid — Incorrect. –\( \ce{NO2} \) is the strongest electron withdrawing group here, so this is the strongest acid, with the lowest pKa (3.41).
NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids
