Question:
Which is a weakest acid among the following ?
Which is a weakest acid among the following ?
Show Hint
Electron donating alkyl groups weaken the acid, and electron withdrawing Cl strengthens it.
Updated On: Oct 1, 2026
- \((\text{CH}_3)_2\text{CHCOOH}\)
- \(\text{CH}_3\text{COOH}\)
- \(\text{ClCH}_2\text{COOH}\)
- \(\text{Cl}_3\text{CCOOH}\)
Show Solution
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The Correct Option is A
Solution and Explanation
Step 1: Understanding the Concept:
A carboxylic acid is stronger if its carboxylate ion is more stable. Electron withdrawing groups (such as Cl) stabilise the anion. Electron donating groups (such as alkyl) destabilise it.
Step 2: Rank:
\(\text{Cl}_3\text{CCOOH}\): three Cl atoms, strongest \(-I\) effect, strongest acid (p\(K_a\) about \(0.65\)).
\(\text{ClCH}_2\text{COOH}\): one Cl, p\(K_a\) about \(2.86\).
\(\text{CH}_3\text{COOH}\): p\(K_a\) about \(4.76\).
\((\text{CH}_3)_2\text{CHCOOH}\): two methyl groups push electrons towards the carboxyl group (\(+I\) effect), making the ion least stable. p\(K_a\) about \(4.86\).
Step 3: Order:
\[ \text{Cl}_3\text{CCOOH} > \text{ClCH}_2\text{COOH} > \text{CH}_3\text{COOH} > (\text{CH}_3)_2\text{CHCOOH} \]
The weakest is option A.
Final Answer:
The weakest acid is \((\text{CH}_3)_2\text{CHCOOH}\), option (A). \[ \boxed{(\text{CH}_3)_2\text{CHCOOH}} \]
A carboxylic acid is stronger if its carboxylate ion is more stable. Electron withdrawing groups (such as Cl) stabilise the anion. Electron donating groups (such as alkyl) destabilise it.
Step 2: Rank:
\(\text{Cl}_3\text{CCOOH}\): three Cl atoms, strongest \(-I\) effect, strongest acid (p\(K_a\) about \(0.65\)).
\(\text{ClCH}_2\text{COOH}\): one Cl, p\(K_a\) about \(2.86\).
\(\text{CH}_3\text{COOH}\): p\(K_a\) about \(4.76\).
\((\text{CH}_3)_2\text{CHCOOH}\): two methyl groups push electrons towards the carboxyl group (\(+I\) effect), making the ion least stable. p\(K_a\) about \(4.86\).
Step 3: Order:
\[ \text{Cl}_3\text{CCOOH} > \text{ClCH}_2\text{COOH} > \text{CH}_3\text{COOH} > (\text{CH}_3)_2\text{CHCOOH} \]
The weakest is option A.
Final Answer:
The weakest acid is \((\text{CH}_3)_2\text{CHCOOH}\), option (A). \[ \boxed{(\text{CH}_3)_2\text{CHCOOH}} \]
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