Concept:The acidity of halogenated carboxylic acids is highly dependent on the Inductive Effect (\( -I \)). Strongly electronegative halogens pull electron density away from the \( \text{O-H} \) bond, weakening it, and simultaneously stabilize the resulting carboxylate anion.
Formula:$$ \text{Acidity} \propto \text{Number of Electron-Withdrawing Groups (EWG)} $$
Solution:- Ethanoic acid has no EWG halogens.
- Chloroethanoic acid has 1 Chlorine atom.
- Dichloroethanoic acid has 2 Chlorine atoms.
- Trichloroethanoic acid possesses 3 Chlorine atoms pulling electron density, generating massive stabilization of the anion, rendering it the strongest acid listed.
Why other options are incorrect:The strength increases strictly sequentially: Unsubstituted < Monochloro < Dichloro < Trichloro.
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