Read the passage carefully and answer the 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)
The stronger an acid, the smaller its pKa. So increasing pKa means decreasing acid strength. A carboxylic acid becomes stronger when an electron-withdrawing group is attached, because it spreads the negative charge of the carboxylate ion by its −I effect. The order of this effect for the groups here is \( \ce{CF3} \) > \( \ce{NO2} \) > CN, while H withdraws nothing.
(A) \( \ce{CF3-COOH} \): three fluorine atoms on the carbon next to –COOH give the strongest −I effect. It is the strongest acid (pKa about 0.2).
(B) \( \ce{O2N-CH2-COOH} \): the nitro group is a strong −I group (pKa about 1.7).
(C) \( \ce{NC-CH2-COOH} \): the cyano group withdraws a little less than the nitro group (pKa about 2.5).
(D) \( \ce{H-COOH} \): no electron-withdrawing group; the weakest acid of the four (pKa about 3.8).
In increasing order of pKa:
\( \displaystyle \ce{CF3COOH} \lt \ce{O2NCH2COOH} \lt \ce{NCCH2COOH} \lt \ce{HCOOH} \)
Shortcut: \( \ce{CF3COOH} \) has the lowest pKa and \( \ce{HCOOH} \) the highest, so the order starts with (A) and ends with (D): Options (1) and (3). Since \( \ce{NO2} \) withdraws more strongly than CN, (B) comes before (C), which leaves Option (1).
NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids – …
