Home > Community > Why is benzoic acid more acidic than p-chlorobenzoic acid?
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+ Resonance
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Misaiel Dominguez

Why is benzoic acid more acidic than p-chlorobenzoic acid?

David Southern  Follow

Where are you getting your information? So far as I can find, 4-chlorobenzoic acid has a $\mathrm{p}K_\mathrm{a}$ of $3.98$ and benzoic is $4.2$.

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Andrew  Follow

No! -COOH is meta directing group and hence it isn't stabilised at the para position. Hence benzoic acid is more acidic than para-chloro benzoic acid(because the resonating structure of benzoic acid is more stable than p-chloro benzoic acid due to the reason mentioned above).

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Jhans Zimner  Follow
The resonance with Benzene deatabilises conjugate bases if you think properly.Resonance is less favoured in P chloro benzoic acid as +ve charge is created near Cl which is unstable due to -I . So electron pair on $ O^-$ can get delocalise very easily in ap chloro benzoic acid rather than benzoic acid. So acidity of Benzoic acid less than p chloro benzoic acid.More
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Jaime Tan  Follow
The meta para comes in external electrophilic attack, not in acidity. For acidity, we have to consider in which case COOH can easily lose its proton. If more electron withdrawing group is attached to COOH its acidity increasesMore
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