Acidity And Electronic Effects

The strength of a substituted acetic acid depends on how well the substituent withdraws electron density and stabilises the carboxylate conjugate base through the inductive effect. Fluorine is the most electronegative halogen, so fluoroacetic acid exerts the strongest -I effect, disperses the negative charge most effectively, and is the strongest acid of the three. Chlorine is less electronegative than fluorine, so chloroacetic acid is a weaker acid than fluoroacetic but still stronger than the unsubstituted acid. Acetic acid has only an electron-donating methyl group, so its carboxylate is least stabilised and it is the weakest. The sequence with acetic first is wrong because the unsubstituted acid cannot be the strongest. The order placing chloroacetic above fluoroacetic contradicts electronegativity trends. The sequence ending with chloroacetic mislabels the weakest acid. This ranking follows NCERT data on halo-substituted acids, where the acid dissociation constant rises steadily as the halogen becomes more electronegative, a trend students are expected to reproduce from first principles rather than memorise. The inductive effect also weakens with distance, so a halogen closer to the carboxyl group would strengthen the acid even more sharply. Plausibility check: acid strength tracks substituent electronegativity here, and since electronegativity decreases F > Cl > H, the acidity must decrease in the same order, confirming the chosen sequence.