Which of the following carboxylic acids will yield the least amount of protons in an aqueous solution?
Answer & explanation
Correct answer: option 4
The correct answer is option 4. 2-methylbutanoic acid.
Let us delve into the factors that influence the acidity of carboxylic acids and analyze why 2-methylbutanoic acid yields the least amount of protons in an aqueous solution compared to the other acids listed.
Acidity of Carboxylic Acids:
The acidity of carboxylic acids is determined by their ability to donate protons (\( \text{H}^+ \)) in an aqueous solution. The more readily a carboxylic acid donates a proton, the stronger the acid. This ability is influenced by the stability of the carboxylate anion (RCOO\(^-\)) formed after deprotonation.
Factors Affecting Acidity:
Inductive Effect:
Electron-withdrawing groups (EWGs) stabilize the carboxylate anion by pulling electron density away from the carboxylate group, making the acid stronger. Electron-donating groups (EDGs) destabilize the carboxylate anion by pushing electron density towards the carboxylate group, making the acid weaker.
Resonance and Hybridization:
Resonance stabilization of the carboxylate anion can also affect acidity, but this is more relevant for aromatic carboxylic acids.
Analysis of Each Carboxylic Acid:
1. Butanoic Acid:
A straight-chain carboxylic acid with no substituents.
Effect: No significant electron-withdrawing or electron-donating effects.
Acidity: Moderately strong acid.
2. Propanoic Acid:
A straight-chain carboxylic acid with no substituents, but with one less carbon than butanoic acid.
Effect: Similar to butanoic acid, but the shorter chain slightly reduces the overall electron-releasing inductive effect of the alkyl chain.
Acidity: Slightly weaker than butanoic acid due to the shorter carbon chain.
3. 2-Chlorobutanoic Acid:
Contains a chlorine atom at the 2-position.
Effect: The chlorine atom is a strong electron-withdrawing group (EWG) via the inductive effect, which stabilizes the carboxylate anion.
Acidity: Significantly stronger acid due to the EWG effect of chlorine, leading to more proton release.
4. 2-Methylbutanoic Acid :
Contains a methyl group at the 2-position.
Effect: The methyl group is an electron-donating group (EDG) via the inductive effect, which destabilizes the carboxylate anion.
Acidity: Weaker acid due to the EDG effect of the methyl group, leading to less proton release.
Conclusion:
2-Chlorobutanoic Acid: Most acidic due to the electron-withdrawing chlorine.
Butanoic Acid: Moderately acidic with no significant substituent effects.
Propanoic Acid: Slightly weaker than butanoic acid due to the shorter chain.
2-Methylbutanoic Acid: Least acidic due to the electron-donating effect of the methyl group.
Therefore, 2-methylbutanoic acid will yield the least amount of protons in an aqueous solution because the electron-donating methyl group reduces its acidity by destabilizing the carboxylate anion.
The correct answer is 2-methylbutanoic acid.