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Q.
Which of the following compound will have the smallest $pK_{a}$ value?
J & K CETJ & K CET 2010Aldehydes Ketones and Carboxylic Acids
Solution:
The stronger is the acid, the lower is the $pK _{ a }$ value. Since, an electron releasing group such as $- CH _{3},- C _{2} H _{5}$ 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.