Question:
Which of the following carboxylic acids has the highest pKa?
Which of the following carboxylic acids has the highest pKa?
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Electron-withdrawing groups like halogens (Cl, F) lower the pKa of carboxylic acids by stabilizing the conjugate base, increasing the acidity of the compound.
Updated On: Mar 11, 2025
ethanolic acid
chloroethanoic acid
fluoroethanoic acid
dichloroethanoic acid
trifluoroethanoic acid
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The Correct Option is A
Solution and Explanation
The pKa of a carboxylic acid is inversely related to its acidity. The lower the pKa value, the stronger the acid. The substituents on the aromatic or alkyl group can influence the acidity of the carboxylic acid. Electron-withdrawing groups (EWGs) like halogens (chlorine, fluorine) typically lower the pKa of carboxylic acids, making them stronger acids by stabilizing the conjugate base (carboxylate ion).
Step 1: Analyzing the compounds
- Ethanolic acid (CH3COOH) has no electron-withdrawing substituents, so its acidity is solely determined by the carboxyl group.
- Chloroethanoic acid (CH2ClCOOH) contains a chlorine atom, which is an electron-withdrawing group. Chlorine will stabilize the conjugate base and make the acid stronger (lower pKa).
- Fluoroethanoic acid (CHF2COOH) contains a fluorine atom, which is an even stronger electron-withdrawing group than chlorine. This makes fluoroethanoic acid even stronger than chloroethanoic acid (lower pKa).
- Dichloroethanoic acid (CHCl2COOH) has two chlorine atoms, which further stabilize the conjugate base, making it stronger than chloroethanoic acid.
- Trifluoroethanoic acid (CF3COOH) contains three fluorine atoms, which are very strong electron-withdrawing groups, making this acid very strong (lowest pKa).
Step 2: Conclusion
Since the presence of electron-withdrawing groups decreases pKa, ethanoic acid, having no electron-withdrawing substituents, has the highest pKa compared to the other options.
Thus, the correct answer is option (A).
Step 1: Analyzing the compounds
- Ethanolic acid (CH3COOH) has no electron-withdrawing substituents, so its acidity is solely determined by the carboxyl group.
- Chloroethanoic acid (CH2ClCOOH) contains a chlorine atom, which is an electron-withdrawing group. Chlorine will stabilize the conjugate base and make the acid stronger (lower pKa).
- Fluoroethanoic acid (CHF2COOH) contains a fluorine atom, which is an even stronger electron-withdrawing group than chlorine. This makes fluoroethanoic acid even stronger than chloroethanoic acid (lower pKa).
- Dichloroethanoic acid (CHCl2COOH) has two chlorine atoms, which further stabilize the conjugate base, making it stronger than chloroethanoic acid.
- Trifluoroethanoic acid (CF3COOH) contains three fluorine atoms, which are very strong electron-withdrawing groups, making this acid very strong (lowest pKa).
Step 2: Conclusion
Since the presence of electron-withdrawing groups decreases pKa, ethanoic acid, having no electron-withdrawing substituents, has the highest pKa compared to the other options.
Thus, the correct answer is option (A).
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