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Why is propenoic acid more acidic than p-methoxybenzoic acid?

I looked online for their $\mathrm{p}K_\mathrm{a}$ values and found them to be $4.37$ and $4.25,$ respectively. The $\mathrm{p}K_\mathrm{a}$ of benzoic acid is $4.20.$ Why does the $\ce{-OMe}$ group make such a big effect on $\mathrm{p}K_\mathrm{a}?$

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    $\begingroup$ Correcting what is probably a typo, the pKa values I found for aqueous solution (Evans' pKa tables) are 4.25 for propenoic acid, 4.2 for benzoic acid, and 4.47 for p-methoxybenzoic acid. However, even with this increased pKa disparity, if anything it's interesting that the values are all so close. Acid strength can span over 80 orders of magnitude, so a 0.3 difference is almost a rounding error, all things considered. $\endgroup$ – Nicolau Saker Neto Apr 13 at 13:06
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    $\begingroup$ Consider the electron donation of the OMe group $\endgroup$ – Waylander Apr 13 at 13:08
  • $\begingroup$ My question is how would you compare these pka values with centainity during an exam? The values are very close for benzoid and propenoic acid. $\endgroup$ – tarzanjunior Apr 13 at 13:10
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    $\begingroup$ Usually, you are only comparing structures of the same class. For example, you would probably just compare the pKa's of propanoic vs. propenoic acid. It would not be reasonable to differentiate between compounds with such similar pKa's on an exam, and I would expect any questions regarding pKa to be more obvious than this. $\endgroup$ – Eli Jones Apr 13 at 13:37
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Why is propenoic acid more acidic than p-methoxybenzoic acid?

This is propenoic acid, $\mathrm{pK_a}=$4.25

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This is p-methoxybenzoic acid, $\mathrm{pK_a}=$4.47

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An electron withdrawing group increases the acidity of a carboxylic acid. It disperses negative charge by inductive/resonance effect and stabilizes the carboxylate ion. Thus, propenoic acid (pKa 4.25) is more acidic than p-methoxybenzoic acid (pKa 4.47).

However the molecular structures are totally different. It seems to me to be comparing apples and oranges! To me such hand waving comparisons between molecules are only meaningful if the two molecules are similar.


The $\mathrm{p}K_\mathrm{a}$ of benzoic acid is $4.20.$ Why does the $\ce{-OMe}$ group make such a big effect on $\mathrm{p}K_\mathrm{a}?$

This is a more interesting question.

\begin{array}{|c|c|} \hline \hline \text{acid} & \text{pka} \\ \hline \text{2-Methoxybenzoic acid} & 4.09 \\ \hline \text{3-Methoxybenzoic acid} & 4.10 \\ \hline \text{Benzoic acid} & 4.19 \\ \hline \text{4-Methoxybenzoic acid} & 4.50 \\ \hline \end{array}

So the phenyl group has about the same effect on the acidity of a carboxylic acid as observed with an ethylene group. However the substitution of other function groups, and their position on the ring can increase or decrease the pKa of the acid.

Data sources:
2-Methoxybenzoic acid
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3-Methoxybenzoic acid
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Benzoic acid
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4-Methoxybenzoic acid
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    $\begingroup$ I agree on the first part, but are you planning to flesh out this second part? Right now you've just listed the pKa's... $\endgroup$ – orthocresol Apr 14 at 7:38

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