Question

What is the strongest acid($p{{K}_{a}}$ value is given)?
(A) $HCOOH$[3.77]
(B) ${{C}_{6}}{{H}_{5}}COOH$[4.22]
(C) $C{{H}_{3}}COOH$[4.7]
(D) $C{{H}_{3}}C{{H}_{2}}COOH$[4.88]

Answer 1

An attempt to this question can be made by drawing the expanded form of the given carboxylic acid. Now determine the strength of each of these compounds based on the groups attached to the acid group. Another way of attempting this question can be by directly comparing the $p{{K}_{a}}$ values given next to the compound.

Complete answer:
Electronic factors are the factor that influence various organic reactions and rearrangements. Electronic effects are significantly observed in organic aromatic compounds. Electronic factors are:
- Inductive effect
- Resonance
- Mesomeric effect
- Electromeric effect
- Hyperconjugation.
$p{{K}_{a}}$ numerically represents the negative log of the dissociation constant value for an acid. The lower the $p{{K}_{a}}$ value, the stronger the acid is considered to be.
$p{{K}_{a}}$ = $-\log ({{K}_{a}})$
Where, ${{K}_{a}}$ stands for the dissociation constant for the acid under consideration.
As suggested in the hint we will compare the $p{{K}_{a}}$ values given next to the compound to determine the strongest acid. The compound, $HCOOH$ has the lowest $p{{K}_{a}}$ value of 3.77. Hence it is the strongest acid among the given options.

Therefore, the correct answer is option (A).

Note: The above question can be solved without the use of $p{{K}_{a}}$ values. The group attached to the carboxylic acid group is the hydrogen atom. Hydrogen does not exhibit any electronic effects and thus does not destabilise the conjugate base of acid. This is the reason $HCOOH$ is the strongest acid among the given options.