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Why is the basicity of hypoboric acid $\ce{H4B2O4}$? Is it $2$ even though it has $4$ hydroxyl groups? enter image description here

My teacher told us that the basicity of this acid is $2$ but I am not remembering the exact reason for it.

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  • $\begingroup$ Note that boric acid is not a Brønsted acid. $\endgroup$ – Loong Feb 11 '17 at 15:06
  • $\begingroup$ Adding to @Loong comment - Boric acid is a Lewis acid - H2O forms adduct with eg. B(OH)3 --> B(OH)4- + H+. Similar will happen with your structure - we are not losing H+ from the structure above to form an anion, we are losing H+ from an adduct of the above in water, which we typically measure pKa in - thus pKa has a degree of solvent dependency. $\endgroup$ – Swedish Architect Feb 11 '17 at 18:54
  • $\begingroup$ @Loong Boric acid can act both as Bronsted and Lewis acid. $\endgroup$ – Mithoron Feb 11 '17 at 23:43
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This is a Lewis acid, not a Brønsted acid. Like boric acid $\ce{B(OH)3}$, which form an adduct $\ce{B(OH)3(H2O)}$ which then lose a proton from the water molecule of the adduct: $$\ce{B(OH)3(H2O) -> B(OH)4- + H+ }$$

Above molecule will do this at each B atom, and so lose 2 $\ce{H+}$ in total, not 4. IE will have: $$\ce{B2(OH)4 + 2H2O -> B2(OH)6^{2-} + 2H+}$$ as the overall reaction. Note $\ce{B(OH)3}$ acts as a Lewis acid due to vacant B p orbital, which can accept lone pair of $\ce{H2O}$ and so form an adduct. From this adduct lose proton, and hence acidic.

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I am not familiar with this interesting-looking compound, but (assuming the structure you gave is correct) my guess is that once you have created a dianion from this acid, going to a trianionic species becomes very unfavorable and you would need a base much stronger than hydroxide to do it. For all practical purposes it would behave as a dibasic acid in water solutions.

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  • $\begingroup$ any reasoning why it will become unfavorable for trianion? I was thinking about Hydrogen bonding that will take place between the H atom and anionic O atom is that correct? $\endgroup$ – Piyush Raut Feb 11 '17 at 14:52
  • $\begingroup$ I think the main reason it is so unfavorable to make the trianion is that you are putting a lot of negative charge into a small volume. Fewer opportunities for hydrogen bonding may also be a factor. $\endgroup$ – iad22agp Feb 11 '17 at 16:41
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    $\begingroup$ Boric acid is a Lewis acid, not a Bronsted acid - we form an adduct of B(OH)3 with water and then lose a proton from the adduct, not from B(OH)3 directly - we have a different mode of reactivity and acidity $\endgroup$ – Swedish Architect Feb 11 '17 at 19:04

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