Read the passage carefully and answer the following questions.
Aldehydes, carboxylic acids and ketones are widespread in the animal and plant kingdoms. They play a pivotal role in various biochemical processes of life. Their presence in nature adds fragrance and flavor. These compounds are widely used in food products, pharmaceuticals, paints, resins and other important product industries. These compounds are prepared by various laboratory methods, which mainly include oxidation, formylation, acylation and reduction. Due to the polar nature of the carbonyl group in aldehydes and ketones, they can exhibit different reactions like nucleophilic addition. They do exhibit redox and various condensation reactions which lead to the formation of various important compounds. On the other hand, the carboxylic acids are mainly prepared by oxidation and hydrolysis of different compounds. The carboxylic acid consists of a carbonyl group and the hydroxy group (attached to the carbonyl carbon atom). This makes it possible for the carboxylic acid to participate in various chemical reactions which involve cleavage of the C-OH bond and the O-H bond along with the reactions involving the complete -COOH group.
Arrange the following in the increasing order of their \( pK_a \) values:
(A) \( CF_3-COOH \)
(B) \( O_2N-CH_2-COOH \)
(C) \( NC-CH_2-COOH \)
(D) \( H-COOH \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B), (C), (D)
Note: None of the given options was correct in the NTA question paper. To provide additional question practice to the students, the question has been slightly modified so that the question has one clear answer.
The \( \mathrm{p}K_a \) of an acid is \( -\log K_a \), so the stronger the acid, the smaller its \( \mathrm{p}K_a \). A carboxylic acid is stronger when its carboxylate ion is more stable. An electron-withdrawing group near the –COOH group pulls electron density away by its −I effect, spreads the negative charge of the carboxylate ion and so increases the acidity.
(A) \( \ce{CF3-COOH} \): three fluorine atoms on the carbon next to –COOH give the strongest −I effect; \( \mathrm{p}K_a \approx 0.23 \).
(B) \( \ce{O2N-CH2-COOH} \): the nitro group is a strong electron-withdrawing group; \( \mathrm{p}K_a \approx 1.68 \).
(C) \( \ce{NC-CH2-COOH} \): the cyano group withdraws electrons less strongly than the nitro group; \( \mathrm{p}K_a \approx 2.46 \).
(D) \( \ce{H-COOH} \): there is no electron-withdrawing group at all; \( \mathrm{p}K_a \approx 3.75 \).
NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids – “… The effect of the following groups in increasing acidity order is Ph < I < Br < Cl < F < CN < NO2 < CF3”