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This is from my textbook:

Carboxylic acids owe their acidity ($\mathrm pK_\mathrm a$ of about $5$) to the resonance-stabilized carboxylate anions formed by deprotonation.

Why are they such weak acids? If the anions are resonance-stabilized, I would expect them to be much stronger acids.

Stated another way: this screenshot of the textbook page shows a representative protonation/deprotonation equilibrium for a carboxylic acid:

textbook screenshot showing reaction

Why is the reaction favoured in the reverse direction if the carboxylate ion is more stable than the reactant?

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    $\begingroup$ Resonance is not all that important in the grand scheme of things. If you really want to focus on just one factor, it should be the H-X bond strength. $\endgroup$
    – orthocresol
    Jun 7, 2018 at 18:42
  • $\begingroup$ Note also that you're asking about the aqueous stability of the anion. Now, solvation spheres and dipole effects and dielectric effects are all in play and make this a very complicated problem. $\endgroup$
    – Zhe
    Jun 7, 2018 at 19:40
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    $\begingroup$ About 4-5 is typical pKa for acid with charge of 1 delocalised over 2 oxygen atoms strong acids need need delocalisation over 3 O atoms. $\endgroup$
    – Mithoron
    Jun 7, 2018 at 20:07

2 Answers 2

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Everything is relative in acid-base reactions; the equilibrium will favor the side with the weaker acid. A carboxylic acid with $\ce{pKa = 5}$ will not protonate $\ce{H2O}$ significantly because $\ce{pKa(H3O+) = -0.7}$, but it will protonate something more basic like $\ce{NH3}$ because $\ce{pKa(NH4+) = 9.24}$.

Note: the pKa of $\ce{H3O+}$ is debated, see What is the pKa of the hydronium, or oxonium, ion (H3O+)? for more discussion.

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Carboxylate anions are certainly not more stable than the parent acid. That's exactly what the pKa tells you: the anion is about 10^5 times less stable than the parent acid. The point of resonance stabilization is just that the factor is only 10^5 and not 10^15 which is what you'd expect for generically ripping an H+ off of an organic OH group.

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  • $\begingroup$ reason for more stability of parent acids?plz explain ....also the bond OH in acids is polar so it can easily diassociate H in H2O ... $\endgroup$
    – Hercules
    Jun 8, 2018 at 2:10
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    $\begingroup$ In the parent acid the presence of the H nucleus allows the electrons in the O-H bond to spread out, reducing their kinetic energy, without sacrificing any potential energy, because when they draw away from the O nucleus they come close to the H nucleus. In the anion, these electrons have to stay closer to the O nucleus to enjoy the same low potential energy, and the greater degree of confinement means their kinetic energy is higher. You can think of it as the intramolecular version of resonance stabilization. $\endgroup$
    – Christopher Grayce
    Jun 8, 2018 at 4:00
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    $\begingroup$ I mean, you're just asking in effect why H2O is more stable than OH- and H+. Because chemical bonds work! Electrons have lower energy when they're in a bond than when they're confined to the atoms that form the bond. $\endgroup$
    – Christopher Grayce
    Jun 8, 2018 at 4:01

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