Acidic strength of para substituted benzoic acids

![4-Nitrobenzoic acid](https://i.stack.imgur.com/UUHqS.png)
![4-Methoxybenzoic acid](https://i.stack.imgur.com/dWTVG.png)


Comparing the acidic strength, the nitro-substituted group is definitely more acidic. This can be reasoned by fact that a higher delta positive charge which develops on the H due to the strong **-M** nature of the nitro group.
Another way to check the acidity is to check the stability of the conjugate base.


The conjugate base would be the carboxylate ion which being a **-M** group creates a lesser electron density at the ortho and para positions. This positive charge is however stabilised by the methoxy group, which is a **+M** group, and destabilised by the nitro group. So from this viewpoint, the methoxy derivative should have been more acidic.


Why is this not so? What is the fault in my reasoning?




You are somewhat on the right track. However, consider that acidity depends on the relative stabilization of the acid vs. the conjugate base, since the expression for $\mathrm{p}K\_\mathrm{a}$ is as follows: $$\mathrm{p}K\_\mathrm{a} = -\log\left(\frac{[\ce{A-}][\ce{H+}]}{[\ce{HA}]}\right)$$


Consider this figure for a visualization:


[![enter image description here](https://i.stack.imgur.com/zSLfI.png)](https://i.stack.imgur.com/zSLfI.png)


Electron-withdrawing groups like *p*-$\ce{NO2}$ (nitro group in the para position) increase acidity by making the carboxylate ion stable relative to the protonated form. However, electron-donating groups like *p*-methoxy decrease acidity relative to standard benzoic acid by destabilizing the (already negatively charged) carboxylate anion!


TL; DR: An electron-withdrawing group like *p*-$\ce{NO2}$ will better stabilize the negatively charged $\ce{COO-}$ group than an electron-donating group like *p*-$\ce{OCH3}$.


I found a neat picture from UC Davis that summarizes what I have said:
![enter image description here](https://i.stack.imgur.com/MQoaS.png)


 10

You thought correct but when you are talking about the acidity by taking into account the conjugate basicity then you have to stabilize the negative charge on the acid group which becomes more stable when you introduce an EWG(electron withdrawing group) in the ring which provides -M/-I to the acid group and hence helps it to delocalize its negative charge and become stable. 
And there is no concern of the acidity of benzoic acid with the partial positive charge appearing on the ortho positions(here) because the proton is being released by the carboxylic group. Therefore to make it stable we need an EWG.


 -4

In Methoxy Benzoic Acid,we have Resonance or mesomeric effect which stabilizes the carboxylate anion.


In Nitro Benzoic Acid, we have Intermolecular Attractions (Some may call it as Hydrogen Bonding) between the Nitrogen(of Nitro group) in one molecule and the Carboxylate Anion in another molecule. 


So, since the Intermolecular Attraction is more stronger than the Resonance effect, the Nitro benzoate is more stabilized than Methoxy Benzoate. So, Nitro Benzoic Acid is more acidic.
![enter image description here](https://i.stack.imgur.com/SOgDW.png)


 -6