A) Benzoic acid
B) Formic acid
C) Acetic acid
D) Phenyl acetic acid
Correct Answer: B
Solution :
The stronger is the acid, the lower is the \[\text{p}{{\text{K}}_{\text{a}}}\]value. Since, an electron releasing group such as \[-C{{H}_{3}},-{{C}_{2}}{{H}_{5}}\]destabilises the carboxylate ion, thus decreases the acidic strength of the acid. Phenyl group is however electron withdrawing but benzoic acid is a weaker acid as compared to formic acid. This is because of + R effect of phenyl group, which on the other hand destabilises the carboxylate ion. Hence, formic acid (HCOOH) is the strongest acid. Thus, its \[p{{K}_{a}}\]value will be lowest.
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