Question

Which of the following compounds is more acidic?
(A) $N{{O}_{2}}-C{{H}_{2}}-COOH$
(B) $N{{O}_{2}}-C{{H}_{2}}-C{{H}_{2}}-COOH$
(C) $Cl-C{{H}_{2}}-C{{H}_{2}}-COOH$
(D) $C{{H}_{3}}-CC{{l}_{2}}-COOH$

Answer 1

Hint: Acidity of a carboxylic acid is directly proportional to the stability of the corresponding conjugate base formed. More stabilized the conjugate base is more is the acidity of the carboxylic acid. Conjugate bases have negative charge.

Complete step by step answer:
Carboxylic acids dissociate in water to form carboxylate ion and the hydronium ion. The carboxylate ion formed is stabilized through resonance by effective delocalization of the negative charge. The carboxylate ion formed is the conjugate base of the carboxylic acid, is stabilized by two equivalent resonance structures in which the negative charge is effectively delocalized between two more electronegative oxygen atoms.
The acidity of the carboxylic acid also depends upon the substituent attached to the carboxyl group whether aryl or alkyl group. When an electron withdrawing group is attached to the carboxyl group, it stabilizes the conjugate base of the carboxyl group, through resonance or inductive effect. This enables effective delocalization and therefore, increases stability.
More electron withdrawing group attached, the more is the acidity of the carboxylic acid.
In option A, the nitro group is attached directly to the $\alpha -carbon$ of the carboxylic group, and also it is more electron withdrawing than Chloride ion, therefore option A will be the most acidic acid.
In option b, the nitro group is attached to the carbon next to $\alpha -carbon$ of the carboxylic group,therefore it will be less acidic than option A.
Therefore, the correct answer is option A.

Note: When an electron donating group is attached to the carboxyl group, it destabilizes the conjugate base of the carboxylic acid, as it donates electrons to the electron rich species, that is to the conjugate base. This decreases the stability of the conjugate base and therefore decreases the acidity of the carboxylic acid.