The correct order of acid character of the following compounds is : I: Phenol, II: p-Nitrobenzoic acid, III: Benzoic acid, IV: p-Toluic acid
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To compare the acidity of substituted benzoic acids, remember: electron-withdrawing groups (EWGs) like -NO₂, -CN, -X increase acidity, while electron-donating groups (EDGs) like -CH₃, -OCH₃, -OH decrease acidity. Carboxylic acids are always stronger acids than phenols.
The acidity of these compounds depends on the stability of the conjugate base formed after donating a proton.
First, compare carboxylic acids (II, III, IV) with phenol (I). Carboxylic acids are significantly more acidic than phenols because the carboxylate anion is stabilized by resonance involving two electronegative oxygen atoms, delocalizing the negative charge more effectively than in the phenoxide ion. So, (II, III, IV)>I.
Next, compare the substituted benzoic acids. The acidity of benzoic acid is affected by the substituent on the benzene ring.
Compound II (p-Nitrobenzoic acid): The nitro group (-NO$_2$) is a strong electron-withdrawing group (-I and -R effect). It stabilizes the carboxylate anion by withdrawing electron density, thereby increasing the acidity.
Compound III (Benzoic acid): This is the reference compound with no substituent.
Compound IV (p-Toluic acid): The methyl group (-CH$_3$) is an electron-donating group (+I and hyperconjugation effect). It destabilizes the carboxylate anion by pushing electron density towards it, thereby decreasing the acidity.
Therefore, the order of acidity for the carboxylic acids is: p-Nitrobenzoic acid>Benzoic acid>p-Toluic acid. (II>III>IV).
Combining all the compounds, the final order of decreasing acid strength is:
II>III>IV>I.
