Question

Which of the following is the most acidic?
(A) \[HCOOH\]
(B) \[C{H_3}COOH\]
(C) \[C{H_3}C{H_2}COOH\]
(D) \[C{H_3}C{H_2}C{H_2}COOH\]

Answer 1

Hint: The acidic strength of carboxylic acid varies on the basis of total power needed to withdraw electrons of the atoms bonded with the carboxyl group. The conjugate base of carboxylic acid that is carboxylate ion is stabilized by the two resonance structures in which the delocalisation of negative charge occurs between two more electronegative oxygen atoms. Thus, these resonance structures are the reason for the acidity of carboxylic acid.

Complete step by step answer:
Carboxylic acid contains a carboxyl group \[\left( {COOH} \right)\]attached to an alkyl or aryl group. Carboxylic acid is also an organic compound.
As the name defines, carboxylic acids are acidic in nature because hydrogen belongs to the \[COOH\] group.
Dissociation of carboxylic acids takes place in water resulting in the formation of hydronium ion and carboxylate ion.

The dissociation of carboxylic acid occurs as:
\[RCOOH + {H_2}O \rightleftharpoons {{H_3}{O^ + }} + {RCO{O^ - }} \]
The formed carboxylate ion can be stabilized by resonance with effective delocalization of negative charge.

Carboxylic acids are the strongest acids in terms of acidity from alcohols and even phenols. Further, the acidity of carboxylic acids depends on the nature of the substituent alkyl or aryl group attached to the carboxyl group.
An electron withdrawing group ensures the effective delocalization of negative charge through resonance or the I – effect. Hence, the electron withdrawing groups increase the stability of the conjugate base that is formed and hence even acidity of carboxylic acid.
An electron – relating group destabilizes the formed conjugate base and hence decreases the acidity.
Although, all the acids in the options are weak acids but in:
Option A: \[H\] has comparatively more electron withdrawing power.
Option B: \[C{H_3}\] has lesser electron withdrawing power than \[H\].
Similarly, option (C) and (D) are much weaker than \[C{H_3}COOH\] and \[HCOOH\]
So, the most acidic is \[HCOOH\].

So, the correct answer is Option A.

Note: The simplest way to understand is that the withdrawal of electrons leads to increase in acidity whereas donation of electrons decreases it. This happens because the electron-withdrawing groups delocalize the charge of the carboxylate ion making it more acidic.