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4-Nitrobenzoic acid 4-Methoxybenzoic acid

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?

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  • $\begingroup$ Your chief consideration should be how the negative charge on the conjugate base is stabilised. It is not very important to consider all these little things relating to the multiple resonance structures of the molecular ions. $\endgroup$
    – Tan Yong Boon
    Jan 27, 2018 at 16:48
  • $\begingroup$ Anyway, it is important to note that both the methoxy group and nitro group are inductively withdrawing. However, the methoxy group is +M while the nitro group is -M and more importantly, note that the nitro groups is meta-directing in EAS, generating partial positive charges on the ortho and para positions. Even if you claim that the carboxylate group is able to generate positive charges on the ortho and para positions, these are the positions relative to it. Note that the position ortho to the carboxylate group is the position meta to the nitro group. $\endgroup$
    – Tan Yong Boon
    Jan 27, 2018 at 16:53
  • $\begingroup$ Thus, the mesomeric withdrawing effect of the nitro group does not destabilise the ion as it does not withdraw via the mesomeric effect from the meta and ipso positions relative to it (i.e. the ortho and para positions relative to the carboxylate group). Thus, your reasoning is flawed. $\endgroup$
    – Tan Yong Boon
    Jan 27, 2018 at 16:55

3 Answers 3

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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

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

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  • $\begingroup$ How does the carboxylate ion generate a negative charge in the ring? Carboxylate groups are supposed to withdraw electron density from the ring. $\endgroup$
    – user117913
    Jul 20, 2015 at 16:14
  • $\begingroup$ Thanks for pointing out 'in the ring'. I've removed that phrase from my answer now. What I mean is that when the COOH is deprotonated, you get a COO- group. The p-NO2 allows a positive charge to develop on the position in the ring at which the COO- group is located. The positive and negative charges are therefore very close together, and this decreased charge separation increases the stability of the conjugate base. $\endgroup$
    – GnomeSort
    Jul 21, 2015 at 8:33
  • $\begingroup$ @GnomeSort I think it is the inductive effect of that ring carbon with positive charge. His reasoning is correct when he breaks both bonds, but even in that situation, the +I of both negative oxygens will dominate. $\endgroup$
    – Deepak Arya
    Jun 23, 2021 at 9:02
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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.

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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

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    $\begingroup$ Those are definitely not hydrogen bonding interactions. $\endgroup$
    – jerepierre
    Jul 9, 2015 at 15:13
  • $\begingroup$ When did i say that they are hydrogen bond? $\endgroup$
    – Sabbarish
    Sep 6, 2015 at 4:04

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