Concept:Acid strength is magnified by electron-withdrawing groups (EWG). While the nitro group and fluorine are stronger individual EWGs than chlorine, the sheer cumulative effect of
three chlorine atoms on one carbon outweighs a single stronger group.
Solution:- Fluoroethanoic acid has 1 fluorine atom (strong single pull).
- Nitroethanoic acid has 1 nitro group (very strong single pull).
- Trichloroethanoic acid features 3 chlorine atoms. Their combined, additive inductive effect (\( -I \)) severely depletes electron density from the carboxylate group.
- Consequently, the trichloroacetate anion is the most stabilized, making the parent acid the strongest (lowest pKa).
Why other options are incorrect:Propanoic acid has electron-donating alkyl groups (weakest acid). Single substituents (Fluoro, Nitro) do not surpass the collective pull of three chlorines.
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