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Acidic strength comparison of squaric acid and rhodizonic acid
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Marcio Da Silva
Acidic strength comparison of squaric acid and rhodizonic acid
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 generic behavior for 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.
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 generic behavior for 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.
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.
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.
@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?More
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 didnt consider structures with charge separation on carbonyl groups - they are important here.More
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 generic behavior for 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.
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 generic behavior for 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.
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.
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