1
$\begingroup$

Firstly, I must preface that I am a Biology student. To elaborate my question, I was looking for the reason why carboxylic acids are more acidic than alcohols, despite both compounds containing —OH groups. From my understanding it was because of the inductive effect, an example of this effect is CH3COOH, shown in the figure below:

enter image description here

The inductive effect in CH3COOH occurs because of the electron deficiency in the carbon atom, C (which has a partial positive charge, δ+). This electron deficiency on the carbon atom is present due to the more electronegative double bonded oxygen atom, O. However, the O single bonded to the C is also more electronegative, as a result, the electron density of the single bonded O moves to the C in order to 'counteract' the δ+ on the C, leaving the H 'less strongly attached' to the O (and therefore, the molecule), thus, increasing its acidity.

Now, in an alcohol there is only one oxygen atom, see the figure below:

enter image description here

Because there is no second O attached to the C, which would cause it to become more positive, the O does not become less positive, by sharing electrons to counteract the positive C. Thus, the hydrogen atom is 'more strongly attached' to the O which is more negative compared to the single bonded oxygen atom in the carboxylic acid.

My question is, why does the carbon atom in the carboxylic acid (first figure) counteract the change and increase its pull for the electrons of the single bonded oxygen atom, instead of becoming more positive?

$\endgroup$
2
  • 1
    $\begingroup$ Compare acidity of these 2 pairs: $\ce{Cl-O-H}$ vs $\ce{O3Cl-O-H}$ and $\ce{R-CH2-O-H}$ vs $\ce{R-CO-O-H}$. Carboxylic acids and perchloric acid have stronger polarization of $\ce{O-H}$ bond by oxygen induction and better delocalication of anionic charge, what stabilizes the anion. $\endgroup$
    – Poutnik
    Commented Jan 13, 2023 at 8:39
  • 2
    $\begingroup$ This thing with acid is rather describing mesomeric effect in a misleading way. $\endgroup$
    – Mithoron
    Commented Jan 13, 2023 at 16:01

1 Answer 1

4
$\begingroup$

That's an interesting reasoning you give here. The point is, the inductive effect is not the major factor explaining the acidity of a compound.

Electronegativity will give insights on the position of electrons along the bond, explaining that the hydrogen atom can be detached by a base in both cases (the lengths of carbon-oxygen bonds also need to be considered to understand the interaction between oxygens and carbon).

Yet, what makes a carboxylic acid more acidic than an alcohol is the stability of the conjugated base : a carboxylate ion, thanks to the $\ce{C=O}$ bond on the same carbon as the $\ce{C-O^-}$, is stable by mesomeric resonance (the lone-pair gained by the oxygen atom releasing the hydrogen can 'move', the direct consequence is that both carbon-oxygen bonds will have the same length, between $\ce{C-O}$ and $\ce{C=O}$). However, the alcoholate will not have this resonance, thus will be less stable (see this link for a longer development on that, and images).

I don't know if this is what you wanted to know, do not hesitate to tell me otherwise.

$\endgroup$
7
  • $\begingroup$ Firstly, I always want to know new things. Secondly, thanks Thomas I appreciate the depth, as a second year Biology student (with one completed introductory chemistry course) I tried my best to correctly interpret your answer and I have read the link, though I lack the proper foundational knowledge required for me to understand the terms and concepts presented in the LibreTexts page. $\endgroup$ Commented Jan 14, 2023 at 12:27
  • $\begingroup$ Nonetheless, from my understanding, the two O atoms in the carboxylate ion (R−C−O₂⁻) (conjugate base of carboxylic acid) can distribute or 'spread out' the charge of -1, whereas the -1 charge is locked on the single O atom in the alcohol ion (R−C−H₂−O⁻) (conjugate base of alcohol acid). Therefore, carboxylate is less polar and more stable than the alcohol ion, since H⁺ will be less attracted to the less negatively charged single bonded O atom in carboxylate, compared to the more negatively charged O atom in the alcohol ion. $\endgroup$ Commented Jan 14, 2023 at 12:27
  • $\begingroup$ However, I have no idea why 'charge delocalisation' and resonance occur. From what I understood, according to resonance theory, the carboxylate can have two structures with a -1 charge on either oxygen atom (no idea how the electron hops from one O atom to the other), these structures are averaged resulting in each oxygen atom having a charge of -0.5, for example. If the single bonded O atom in one structure of carboxylate had the -1 charge then wouldn't carboxylate be just as stable as an alcohol ion? $\endgroup$ Commented Jan 14, 2023 at 12:27
  • $\begingroup$ Obviously, my lack of chemistry background is involved here. Lastly, I do not understand how this answers my initial question on the inductive effect. $\endgroup$ Commented Jan 14, 2023 at 12:28
  • 1
    $\begingroup$ “ I have no idea why 'charge delocalisation' and resonance occur”: it is a matter of bonds and electrons. Compared to a single bond with electrons following the inter-atom axis, the double bond is a conjunction of both a single bond, and an extra pair of electrons less strongly fixed in between atoms. That explains why these two extra electrons can move more freely from one bond to another if they can (standard bond number rules still apply). Hence, the lone pair giving the -1 charge to the alcoholate can form a double bond only if the other bond gives -1 to the other oxygen. $\endgroup$ Commented Jan 15, 2023 at 15:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.