'Ka' values of a few organic acids are given. The order of acid strength is:
Correct answer: A. CCl3COOH > CHCl2COOH > CH2ClCOOH > CH3COOH
- A. CCl3COOH > CHCl2COOH > CH2ClCOOH > CH3COOH
- B. CH3COOH > CHCl2COOH > CCl3COOH > CH2ClCOOH
- C. CHCl2COOH > CH3COOH > CCl3COOH > CH2ClCOOH
- D. CCl3COOH > CH3COOH > CHCl2COOH > CH2ClCOOH
Explanation
CCl3COOH > CHCl2COOH > CH2ClCOOH > CH3COOH: This option suggests that trichloroacetic acid (CCl3COOH) is the strongest acid, followed by dichloroacetic acid (CHCl2COOH), chloroacetic acid (CH2ClCOOH), and acetic acid (CH3COOH). The order is based on the increasing number of chlorine atoms attached to the carboxyl group, which increases the electron-withdrawing effect and enhances the acidity of the molecule. Therefore, this option represents the correct order of acid strength.
This question appeared on the UHS MDCAT 2015 paper, which you can sit online with every answer explained.
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About Reactivity of Carboxylic Acids
Carboxylic acids show acidic behaviour because their carboxylate ions are resonance-stabilised, and electron-withdrawing groups can change their strength. Reactions include neutralisation, esterification, formation of acid chlorides and amides, reduction, decarboxylation and substitution at the acyl carbon, which differs from the reactions of phenols.
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