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Why m-hydroxybenzoic acid has higher acidity than p-hydroxybenzoic acid?

I have a reasoning that since hydroxy group is an activating group, then it increases the electron density on its para position. So for p-hydroxybenzoic acid, the carbonyl carbon has a greater ability to share molecular orbital's electron cloud to its attached hydroxy group. So the hydroxy group doesn't release $\ce{H+}$ easily. But that's not the case for m-hydroxybenzoic acid.

Is it right?

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The -$\ce{OH}$ group has two effects, namely +R and -I effect i.e. Resonance and Inductive effect. When -$\ce{OH}$ group is attached to the para position, it has a tendency to delocalize it's electrons towards the aromatic ring (i.e., benzene). Thus the electron density on oxygen atoms of carboxylic group increases and the polarity between $\ce{O}$ and $\ce{H}$ decreases leading to less acidity. Whereas when the -$\ce{OH}$ group is attached to the meta position, +R effect is not applicable and only -I effect is effective. Thus it withdraws the electrons from the ring, and increasing the polarity between $\ce{O}$ and $\ce{H}$ in carboxylic group, making it more acidic.

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  • $\begingroup$ That's what I said+ something $\endgroup$ – Mockingbird Feb 5 '17 at 6:53

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