Question:
Which of the following has highest pKa value?
Which of the following has highest pKa value?
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In general, the more electron-withdrawing groups attached to the carboxyl group, the lower the pKa and the stronger the acid. Fluorine, CN, and NO\(_2\) are strong electron-withdrawing groups.
Updated On: Apr 18, 2026
- CH\(_3\)COOH
- F-CH\(_2\)-COOH
- CN-CH\(_2\)-COOH
- Cl-CH\(_2\)-COOH
- NO\(_2\)-CH\(_2\)-COOH
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The Correct Option is A
Solution and Explanation
Step 1: Understanding pKa value.
pKa is a measure of the acidity of a compound, with a lower pKa indicating a stronger acid. The presence of electron-withdrawing groups (EWGs) typically lowers the pKa, making the acid stronger.
Step 2: Analysis of the options.
\begin{itemize} \item (A) CH\(_3\)COOH: This is acetic acid, which is a weak acid. It has the highest pKa among the given options, indicating that it is the weakest acid. \item (B) F-CH\(_2\)-COOH: The presence of the electronegative fluorine atom withdraws electron density from the carboxyl group, making it a stronger acid and lowering its pKa value. \item (C) CN-CH\(_2\)-COOH: The nitrile group (CN) is an electron-withdrawing group, which lowers the pKa further, making this compound a stronger acid. \item (D) Cl-CH\(_2\)-COOH: Chlorine is also an electron-withdrawing group, but it is less effective than fluorine or nitrile groups in lowering the pKa. \item (E) NO\(_2\)-CH\(_2\)-COOH: The nitro group (NO\(_2\)) is a very strong electron-withdrawing group, which significantly lowers the pKa value, making this the strongest acid. \end{itemize}
Step 3: Conclusion.
Therefore, the compound with the highest pKa value is acetic acid (CH\(_3\)COOH), as it has the weakest electron-withdrawing effect and is the least acidic.
Final Answer: CH\(_3\)COOH.
pKa is a measure of the acidity of a compound, with a lower pKa indicating a stronger acid. The presence of electron-withdrawing groups (EWGs) typically lowers the pKa, making the acid stronger.
Step 2: Analysis of the options.
\begin{itemize} \item (A) CH\(_3\)COOH: This is acetic acid, which is a weak acid. It has the highest pKa among the given options, indicating that it is the weakest acid. \item (B) F-CH\(_2\)-COOH: The presence of the electronegative fluorine atom withdraws electron density from the carboxyl group, making it a stronger acid and lowering its pKa value. \item (C) CN-CH\(_2\)-COOH: The nitrile group (CN) is an electron-withdrawing group, which lowers the pKa further, making this compound a stronger acid. \item (D) Cl-CH\(_2\)-COOH: Chlorine is also an electron-withdrawing group, but it is less effective than fluorine or nitrile groups in lowering the pKa. \item (E) NO\(_2\)-CH\(_2\)-COOH: The nitro group (NO\(_2\)) is a very strong electron-withdrawing group, which significantly lowers the pKa value, making this the strongest acid. \end{itemize}
Step 3: Conclusion.
Therefore, the compound with the highest pKa value is acetic acid (CH\(_3\)COOH), as it has the weakest electron-withdrawing effect and is the least acidic.
Final Answer: CH\(_3\)COOH.
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