# Basicity of hypoboric acid

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

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

• Note that boric acid is not a Brønsted acid. – Loong Feb 11 '17 at 15:06
• 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. – Swedish Architect Feb 11 '17 at 18:54
• @Loong Boric acid can act both as Bronsted and Lewis acid. – Mithoron Feb 11 '17 at 23:43

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.