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In p-anisic acid the phenyl ring with methoxy group acts as strong electron donating group by mesomeric effect (+M), whereas in acetic acid there is only an electron donating nature by inductive effect.

According to this, acetic acid should me more acidic compared to p-anisic acid, but it turns out that p-anisic acid is significantly more acidic:

Acid Structure $\mathrm{p}K_\mathrm{a}(\pu{25 °C})$
Acetic acid Structure of acetic acid 4.76
p-Anisic acid Structure of p-anisic acid 4.47

Why is p-anisic acid more acidic and what causes a significant difference in the $\mathrm{p}K_\mathrm{a}$ values?

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    $\begingroup$ An sp^2 hybridized carbon atom is more electronegative than an sp^3 hybridized carbon atom. The carboxyl group is attached to sp^2 carbon in p-Anisic acid whereas it is attached to an sp^3 hybridized carbon in acetic acid which can be a contributing factor in p-Anisic acid being more acidic. $\endgroup$ Feb 8, 2022 at 11:01
  • $\begingroup$ A better way to interpret this would be that a stronger acid yields a more stable conjugate base upon losing an H+ ion. In the C.B. of these acids, the carboxyl group gets converted to COO-. In 4-methoxybenzoate(C.B of p-Anisic acid), this negative charge on COO- undergoes resonance thereby distributing electron density throughout the ring. In acetate ion, the negative charge remains localized and the +I effect from the methyl group makes it more unstable. Therefore, the C.B of p-Anisic acid is more stable than that of acetic acid and thus, p-Anisic acid is more acidic than Acetic Acid. $\endgroup$ Feb 8, 2022 at 11:10
  • $\begingroup$ @Krish Ratta: Understandably, benzoic acid (4.20) and p-nitrobenzoic acid (3.41) are less acidic than p-anisic acid (4.47). What one doesn't know is the resonance effect of the MeO/NO2 groups relative to the sp2 vs. sp3 argument. Your argument holds because you were given the answer. A priori, p-anisic acid might have been argued to be less acidic than acetic acid (4.76). $\endgroup$
    – user55119
    Feb 9, 2022 at 22:24
  • $\begingroup$ @user55119 Agreed. No theoretical argument can enjoy precedence over pKa values and the best evidences infact are the experimental observations and pKa values. Theory is just an explanation to satisfy ourselves as to why we got the observed result. So it is always fair to argue in this domain and to get confused between acidities or such factors because theoretically, we do not know which effect dominates and why just by only seeing the structure of the compound. And yeah, knowing the answer surely helped me plot out a reason for p-Anisic acid being more acidic. $\endgroup$ Feb 10, 2022 at 7:34
  • $\begingroup$ @user55119 I would also like to inform you about a slight error that creeped into your comment. As per the values furnished by yourself, Benzoic acid and p-nitrobenzene are more acidic than p-Anisic acid and not less acidic and the sp^2 or sp^3 argument doesn't hold here because all these 3 compounds that you are comparing have a benzene ring and a COOH attached to it. Only the groups attached at the para position are different which would make comparison of their acidities(by M and I effect) much simpler compared to the one in the O.P's question $\endgroup$ Feb 10, 2022 at 7:44

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