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p-Methoxybenzoic acid $(\mathrm{p}K_\mathrm{a} = 4.47)$ is a stronger acid than acetic acid $(\mathrm{p}K_\mathrm{a} = 4.76)$ despite the presence of an electron donating group. I understand this is due to the greater electronegativity of $\mathrm{sp^2}$ carbon to which the carboxyl carbon is attached.

But is there a substituent with strong enough donating nature to make benzoic acid $(\mathrm{p}K_\mathrm{a} = 4.20)$ less acidic than acetic acid? If there are, which one might be just strong enough?

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But is there a substituent with strong enough donating nature to make benzoic acid less acidic than acetic acid?

Yes, we can make benzoic acid less acidic than acetic acid by simply substituting strong electron donating groups like $\ce{-N(CH3)2}$ to the para position of benzoic acid. By substituting $\ce{-N(CH3)2}$ to the para position of benzoic acid we can reduce it's $\mathrm{p}K_\mathrm{a}$ to $6.03$! Which is much less acidic than acetic acid.

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    $\begingroup$ That's good to know, but considering -N(CH3)2 gets the pKa all the way to 6.03, there's probably a substituent that would make it just a little less acidic than acetic acid, like I mentioned in the 2nd part of the question. Do you have any idea about such a substituent? Or is there none, at least within the scope of a particularly hard college entrance exam? $\endgroup$
    – Skahoot
    May 24, 2022 at 8:22
  • $\begingroup$ @Skahoot p-aminobenzoic acid aka PABA has pKa for carboxyl 4.88, what is slighly less acidic than 4.75(4.76) for acetic acid. $\endgroup$
    – Poutnik
    May 24, 2022 at 10:49
  • $\begingroup$ This isn't a best example - amino groups complicate things - this 6 is pKa of zwitterion - N-acid. $\endgroup$
    – Mithoron
    May 24, 2022 at 13:40
  • $\begingroup$ @Mithoron, if so, could you mention a more suitable alternative? Although, TBH I think what has been mentioned would be adequate for my level. $\endgroup$
    – Skahoot
    May 24, 2022 at 15:11
  • $\begingroup$ @Skahoot Maybe trimethylammonium, or -BH2 group... $\endgroup$
    – Mithoron
    May 24, 2022 at 19:21

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