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The reaction of carbon dioxide with water yields carbonic acid to some extent (0.4 %) depending on pressure (and temperature).

This process, according to @ron, goes through a four-membered transition stage in which a proton transfer occurs, thereby relieving the unstable "carbonic acid" or hydrated carbon dioxide of its charge separation (The instability of hydrated carbon dioxide or "carbonic acid").

enter image description here

Now, the resulting species is a di-enol; we have hydroxyl groups attached to a carbon that bears a pi bond. This species is likely highly unstable due to the highly positive central carbon atom. On the other hand, neither hydroxyl group makes for a good leaving group, so I can't think of any "forward" (rather than reverse) pathways to more stable products. Also, I can't see the di-enol tautomerizing into a keto. Or can it?

Also, I was arguing that this species is analogous to cis-nitrous acid, in that both species involve a four-membered ring (or at least a psuedo four-memebred ring in the case of cis-HONO). Why does nitrous acid exhibit (unexpectedly) different bond angles?

My points were:

  • Proton transfer within the charge-separated form of carbonic acid mirrors the electrostatic attraction between the hydrogen and the terminal oxygen of cis-HONO.
  • Because the extent of this proton transfer is 0.4%, this suggests that forming a four membered ring is unfavorable (including the case of cis-HONO). Well, of course.
  • This more importantly suggests that any interaction between any terminal oxygen and hydrogen in a four-membered ring is unfavorable); just look at the extent of proton transfer between an acidic oxygen bearing a positive formal charge of and an oxygen bearing a negative formal charge. In isolation, we would expect the below to be highly favorable (at least thermodynamically):

$\ce{RO^+H + RO^- ->RO + ROH}$

  • My professor, however, countered that the resulting product, while not charge separated, is a "di-enol". This is highly unstable; the carbon has a strong partial positive charge. So this distorts the comparison I'm trying to draw between cis-HONO and carbonic acid; the sheer instability of the resulting product from the intramolecular reaction may be hiding any thermodynamic gains to be had from the proton transfer, and thus does not rule out the possibility that the net result of hydrogen bonding in cis-HONO is unfavorable, and that H-bonding is cis-HONO is unable to close up the ONO bond angle.
  • The "di-enol" also reminds me of acid/acyl chlorides, which are stable enough to be isolated, but not exceedingly stable to be completely unreactive. Are acid chlorides a good analogy to carbonic acid? In both cases the central carbon bears a strong partial positive charge.
  • In addition, the charge-separated form is prone to disproportionation to carbon dioxide gas and water, which is highly favorable from an entropic standpoint.

Questions

  1. Is carbonic acid a "di-enol" (if that even is a thing)?
  2. Can true (non-charge separated) carbonic acid tautomerize into more stable forms?
  3. Does this system provide a good parallel with regard to the case of cis-HONO?
  4. Are acid chlorides a good analogy to carbonic acid? In both cases the central carbon bears a strong partial positive charge.
  5. Is true carbonic acid stable?
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  • $\begingroup$ It would be helpful if you included the links to 1) the "according to @ron" answer (cuz I don't remember advocating a 4-membered TS) and 2) the HONO question you're referring to. $\endgroup$ – ron Sep 14 '14 at 23:52
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    $\begingroup$ chemistry.stackexchange.com/questions/16076/… $\endgroup$ – Dissenter Sep 14 '14 at 23:55
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    $\begingroup$ chemistry.stackexchange.com/questions/15558/… $\endgroup$ – Dissenter Sep 14 '14 at 23:55
  • $\begingroup$ I'm still missing where I mention a 4-membered TS $\endgroup$ – ron Sep 14 '14 at 23:57
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    $\begingroup$ That was implied when you agreed with my proton transfer mechanism. $\endgroup$ – Dissenter Sep 15 '14 at 0:01
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1.Is carbonic acid a "di-enol" (if that even is a thing)?

I think most folks would understand what you mean, di-onal (alkene vs. ketone), since it is a carbonyl double bond, not a carbon-carbon double bond, that is involved, might be preferred.

2.Can true (non-charge separated) carbonic acid tautomerize into more stable forms?

It can undergo keto-onol tautomerization to regenerate itself, but now with a different oxygen involved in the carbonyl and a different oxygen in one of the hydroxyls. I can also imagine a tautomer with a 2-oxygen, 3-membered ring, but I doubt this tautomer is more stable.

3.Does this system provide a good parallel with regard to the case of cis-HONO?

My first reaction is that it is not a close analogy.

  • The carbonic acid situation involves proton transfer. In this case with water around, the proton transfer is likely intermolecular, involving nearby water molecules.

  • The nitrous oxide problem involved a static, intramolecular hydrogen bond

4.Are acid chlorides a good analogy to carbonic acid? In both cases the central carbon bears a strong partial positive charge.

My first reaction here was - no way. But the more I think about it, the more I see your point. They both react via similar mechanisms, you can draw that extra resonance structure with hydroxide or chloride ionized for both of them, you have to protect them both in order to store them. Guess I'd say there are a number of analogies between the two compounds.

5.Is true carbonic acid stable?

Yes, when stored under the appropriate conditions (just like an acid chloride). To compare it's kinetic stability to some other compound, we'd need to find out what the activation energy for decomposition is. To compare it's thermodynamic stability, we'd need to calculate it's heat of formation and compare that to the heat of formation of water and carbon dioxide. P.S What is untrue carbonic acid?

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    $\begingroup$ "My first reaction is that it is not a close analogy." What about the dihedral angle of gaseous HOOH and the ONO bond angle of cis-HONO? @ron (that was my professor's analogy; rationalize the dihedral angle of HOOH and that'll explain cis-HONO's ONO bond angle apparently). $\endgroup$ – Dissenter Sep 15 '14 at 0:22
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    $\begingroup$ Also what's an "onol"? Google doesn't turn up anything. I understand it's related to ketones though .. $\endgroup$ – Dissenter Sep 15 '14 at 0:24
  • $\begingroup$ Do you mean carbonic acid instead of HOOH, and which dihedral angle? $\endgroup$ – ron Sep 15 '14 at 0:24
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    $\begingroup$ I mean HOOH, and this dihedral angle: i.stack.imgur.com/VquYQ.png. Also I added two more questions to my OP :D. $\endgroup$ – Dissenter Sep 15 '14 at 0:25
  • $\begingroup$ enols involve a C=C bond, this involves a C=O double bond; so not "en" (as in alkene), but rather "on" (as in ketone). $\endgroup$ – ron Sep 15 '14 at 0:26

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