The carboxylic acid with the highest pK_{a} a | vedclass

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(D) The acidity of substituted benzoic acids depends on the electronic effects of the substituents. Electron-withdrawing groups $(EWG)$ increase acidi

Vedclass · Accepted Answer

$III, IV$

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(D) The acidity of substituted benzoic acids depends on the electronic effects of the substituents. Electron-withdrawing groups $(EWG)$ increase acidity (lower $pK_{a}$),while electron-donating groups $(EDG)$ decrease acidity (higher $pK_{a}$).$1$. $III$ ($p$-methylbenzoic acid): $-CH_3$ is an $EDG$ ($+I$ and hyperconjugation effect),which decreases acidity. Thus,it has the highest $pK_{a}$.$2$. $I$ ($p$-iodobenzoic acid): $-I$ is a weak $EWG$ ($-I$ effect),which slightly increases acidity.$3$. $II$ ($p$-cyanobenzoic acid): $-CN$ is a strong $EWG$ ($-I$ and $-M$ effects),which significantly increases acidity.$4$. $IV$ ($p$-nitrobenzoic acid): $-NO_2$ is a very strong $EWG$ ($-I$ and $-M$ effects),which increases acidity the most. Thus,it has the lowest $pK_{a}$.Therefore,the highest $pK_{a}$ is for $III$ and the lowest $pK_{a}$ is for $IV$. The correct option is $D$.

The carboxylic acid with the highest $pK_{a}$ and lowest $pK_{a}$ values among the following are respectively:

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