Chemistry Carboxylic Acids MDCAT 2016
PMDC Verified Question 78 of 94
'Ka' values of few organic acids are given.
\( \text{CH}_3\text{COOH} = 1.85 \times 10^{-5} \)
\( \text{CCl}_3\text{COOH} = 2.3 \times 10^{-2} \)
\( \text{CHCl}_2\text{COOH} = 5.0 \times 10^{-3} \)
\( \text{CH}_2\text{ClCOOH} = 1.3 \times 10^{-3} \)

The order of the acid strength is:
A
\( \text{CCl}_3\text{COOH} > \text{CHCl}_2\text{COOH} > \text{CH}_2\text{ClCOOH} > \text{CH}_3\text{COOH} \)
B
\( \text{CH}_3\text{COOH} > \text{CHCl}_2\text{COOH} > \text{CCl}_3\text{COOH} > \text{CH}_2\text{ClCOOH} \)
C
\( \text{CHCl}_2\text{COOH} > \text{CH}_3\text{COOH} > \text{CCl}_3\text{COOH} > \text{CH}_2\text{ClCOOH} \)
D
\( \text{CCl}_3\text{COOH} > \text{CH}_3\text{COOH} > \text{CHCl}_2\text{COOH} > \text{CH}_2\text{ClCOOH} \)
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Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: \( \text{CCl}_3\text{COOH} > \text{CHCl}_2\text{COOH} > \text{CH}_2\text{ClCOOH} > \text{CH}_3\text{COOH} \)
Concept:

Acid strength is directly proportional to the acid dissociation constant (\( \text{K}_a \)). A larger \( \text{K}_a \) implies a stronger acid. Furthermore, electron-withdrawing groups (like chlorine) stabilize the conjugate base via the inductive effect (\( -I \)), increasing acid strength.

Formula:

$$ \text{Higher K}_a = \text{Stronger Acid} $$

Solution:

  • Compare the given \( \text{K}_a \) values directly:


  • \( \text{CCl}_3\text{COOH} \): \( 2.3 \times 10^{-2} \) (Largest, strongest)


  • \( \text{CHCl}_2\text{COOH} \): \( 5.0 \times 10^{-3} \)


  • \( \text{CH}_2\text{ClCOOH} \): \( 1.3 \times 10^{-3} \)


  • \( \text{CH}_3\text{COOH} \): \( 1.85 \times 10^{-5} \) (Smallest, weakest)


Why other options are incorrect:

The other options arrange the acids contradicting their experimental \( \text{K}_a \) values and the fundamental inductive effect (more halogens = stronger acid).

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