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These two are dibasic acids with the following structures (squaric acid and rhodizonic acid in order).

squaric acid rhodizonic acid

Empirical evidence proves that squaric acid is more acidic than rhodizonic acid.

It is a fact that in most cases the compound whose conjugate base is more stable is more acidic than the other. While comparing the anions of the two, I assumed that since rhodizonic acid would have more equivalent resonating structures, it must be more stable.

So where did I go wrong in my method of comparison?

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  • $\begingroup$ @Mithoron could you please elaborate your point, as a high-school student who has just started studying organic chemistry, I was taught it is one of the major factors while comparing stability, especially if they are equivalent resonating structures. $\endgroup$ Oct 14, 2021 at 5:57
  • $\begingroup$ @Mithoron I came across the structure of croconic acid and its acidic strength "quite fits" into the expected order, i.e. is more than that of squaric acid. But why do we have that "number of mesomeric structures" criteria fails for rhodizonic acid. Can we consider the more number of oxygen atoms to be responsible for this? $\endgroup$ Oct 14, 2021 at 16:09

2 Answers 2

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In neutral rhodizonic acid water solution, not all carbonyl groups are available for delocalization. Some are hydrated i.e. there are gem-diol ($\ce{C(OH)2}$) groups instead of carbonyls (1). This may be surprising, but it's a generic behavior of compounds with conjugated carbonyl groups and electron-withdrawing groups bound to $\ce{C=O}$ in general. Because of that cyclohexanehexone wasn't even detected, while it's "hydrate", dodecahydroxycyclohexane was made over one and a half century ago!

Why then lower analogs, like squaric acid, aren't hampered like that? Having less carbonyl groups can be indeed better - endiol group has big stabilising effect - it donates lots of electron density and makes other C-C bonds partially double. One could employ some of Michael Lautman's reasoning - only for neutral molecule, not conjugated bases - while carbonyls next to endiol group are directly conjugated, further ones are "more vulnerable" to being hydrated.

(1) Gelb, R. I.; Schwartz, L. M.; Laufer, D. A. (1978). "The structure of aqueous rhodizonic acid". Journal of Physical Chemistry. 82 (18): 1985–1988. doi:10.1021/j100507a006.

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I would suggest that you check your work. If you look at the image below, only the the two oxygens marked with an asterisk are accessible by resonance. This means that the resonance hybrid of has the negative charges evenly distributed across all four oxygen atoms. enter image description here

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    $\begingroup$ So how do we get the negative charge in the top oxygen atom? $\endgroup$ Oct 13, 2021 at 21:06
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    $\begingroup$ Deprotonate the other oxygen atom. $\endgroup$ Oct 13, 2021 at 21:47
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    $\begingroup$ I should cut you some slack, as what you drawn was also one of my ideas in here, but dianion is symmetric, fully delocalised and can even be considered aromatic. You didn't consider structures with charge separation on carbonyl groups - they are important here. $\endgroup$
    – Mithoron
    Oct 13, 2021 at 22:53
  • $\begingroup$ Of course... I was trying to suggest the thinking pattern, rather than giving the answer. $\endgroup$ Oct 14, 2021 at 0:20
  • $\begingroup$ @MichaelLautman I came across the structure of croconic acid and its acidic strength "quite fits" into the expected order, i.e. is more than that of squaric acid. But why do we have that "number of mesomeric structures" criteria fails for rhodizonic acid. Can we consider the more number of oxygen atoms to be responsible for this? $\endgroup$ Oct 14, 2021 at 16:12

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