[Super Newb]

Given the following pKa value:

Benzoic acid: 4.2
m-hydroxybenzoic acid:4.1
m-methoxybenzoic acid:4.1
o-methoxybenzoic acid: 4.1

Find the explanation for the observation above.

It is known that the aromatic group of the benzoic acid gives rise to the stabilization of the anion acid group due to the mesomeric effect. Hence, the benzoic acid will be more acidic if the mesomeric effect can be broken by either placing a bulky group on the ortho- position to prevent the formation of co-planar structure between the aromatic ring and the carboxylic group, in this way, the pi-electron in the aromatic group cannot be delocalized to the carboxylic group to stabilized the anion acid group.

Now the problem arise in explaining why the meta- substitute benzoic acid exhibit similar pKa value. Some people argue that the meta substituted benzoic acid is less acidic due to the inductively electron withdrawing capacity of the hydroxy/methoxy group destabilizing the mesomeric form of the benzoic acid. But I feel that the meta substituted benzoic acid is even capable of breaking the conjugation between the aromatic group and carboxylic group by the mechanism as shown below:

Can someone kindly comment if it is possible to disrupt the conjugation between the aromatic group and the carboxylic acid group as shown. Any help will be greatly appreciated.

[mjc123]

No, that is impossible. You can tell something is wrong because the charges don't add up - the total charge is zero for structures 1, 2 and 3, +2 for structure 4 and +1 for structure 5.
Going from 3 to 4 you are pushing electrons onto an O^- - there is no room for that. 4 should have a double bond between C1 and C2, and a 2- on the carbonyl O (assuming that's possible, which it isn't). 4 to 5, you are taking electrons away from an O⁺ - is that likely? 5 should have (in addition to the corrections for 4) a double bond between C3 and C4, and a 2+ on the hydroxyl O (not C3). Structures 4 and 5 are impossible (as resonance structures of m-hydroxybenzoic acid), so m-OH does not mesomerically affect the acidity of the acid.
You need to revise the rules for forming resonance structures.

[Super Newb]

Thank you mjc123 for the comment and spotting the mistake in my drawing of mesomeric structures. However, it has always confounded me on why the inductive effect can impact the acidity of the carboxylic acid with the similar impact as the mesomeric effect–– by decreasing the pKa value by 0.1. I would have expected the decrease to be even more marginal than one that is attributed to mesomeric effect. Could someone kindly direct me in the right direction and give my some insight on the interpretation. Thanks!!

[AdiDex]

decreasing the pKa value by 0.1. I would have expected the decrease to be even more marginal than one that is attributed to mesomeric effect.

You are comparing which compounds , I got confused , please elaborate little .