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)
Core Concept
Electron withdrawing groups increase acidity by stabilising the conjugate base through –I effect.
Stronger electron withdrawing group → stronger acid → lower pKa
Detailed Explanation
CF₃–COOH
CF₃ is a very strong electron withdrawing group due to three fluorine atoms.
Maximum stabilisation of conjugate base → strongest acid → lowest pKa
O₂N–CH₂–COOH
NO₂ is a strong electron withdrawing group.
Significant stabilisation of conjugate base
NC–CH₂–COOH
CN group is electron withdrawing but weaker than NO₂
H–COOH
No electron withdrawing group
Least stabilisation
Weakest acid → highest pKa
Increasing order of pKa (lowest to highest):
CF₃–COOH < O₂N–CH₂–COOH < NC–CH₂–COOH < H–COOH
(A) < (B) < (C) < (D)
Correct option is 1