Question

The relative acidity between carboxylic acids has been used as a means for determining the electron-withdrawing or electron-donating abilities of substituents.

a) The pKa of phenyl acetic acid is 4.31, making it a stronger acid than acetic acid. Is the Ph group electron-donating or electron-withdrawing in this case?

b) The Ph group is a mild o,p-director in EAS reactions. Is the Ph group electron-donating or electron-withdrawing? If the answer contradicts your answer from part a, provide an explanation.

c) 4-Methoxybenzoic acid is a weaker acid than benzoic acid, yet methoxyacetic acid is a stronger acid than acetic acid. Rationalize these seemingly contradictory results.

d) Methyl groups are usually considered electron-donating groups. Propanoic acid is a weaker acid than acetic acid. Is that result in agreement?

Answer 1

a..The pKa of phenyl acetic acid is 4.31, making it a stronger acid than acetic acid. Is the Ph group electron-donating or electron-withdrawing in this case?

Ans- Phenyl is electron withdrawing group in this case, which makes phenyl acetic acid more stronger.

b) The Ph group is a mild o,p-director in EAS reactions. Is the Ph group electron-donating or electron-withdrawing? If the answer contradicts your answer from part a, provide an explanation.

Ans- A phenyl group, I believe would activate by donating electrons. Ortho, para directing groups are electron-donating groups; meta directing groups are electron-withdrawing groups. The halide ions, which are electron-withdrawing but ortho, para directing, are the exception.

c) 4-Methoxybenzoic acid is a weaker acid than benzoic acid, yet methoxyacetic acid is a stronger acid than acetic acid. Rationalize these seemingly contradictory results.

Ans-

Electron-withdrawing groups, like −CN and −OCH3, inductively withdraw electron density, which stabilizes the resulting carboxylate anion and, thus, increases the acidity of the carboxylic acid. These inductive effects are strongly dependent on distance. The −CN and −OCH3 substituents are closer to the carboxylate in the substituted acetic acids than in the substituted benzoic acids, so their effect is greater.

d) Methyl groups are usually considered electron-donating groups. Propanoic acid is a weaker acid than acetic acid. Is that result in agreement?

Ans-

Propanoic acid has one more CH2 than acetic acid that donates electron density to the carboxylate oxygens through the sigma bond backbone of the molecule. The extra electron density makes the acidic proton more reluctant to leave propanoic acid than acetic acid. Hence, propanoic acid is less acidic.