Why does phosphinic acid not form pyroacids?

Why does $\ce{H3PO2}$ not form pyroacids? All I know is pyroacids are derived oxyacids obtained by removing one $\ce{H2O}$ molecule from two molecules of oxyacids. Correct me if this is wrong.


[![Phosphinic acid](https://i.stack.imgur.com/d3YeR.png)](https://i.stack.imgur.com/d3YeR.png)




It’s essentially both reasons mentioned in the comments.


1. “Pyro-” means “fire” or, more generally, “heat” (“**pyro**technics” = “**fire**works”). Hypophosphorous acid, or more accurately phosphinic acid, is thermally not very stable decomposing at 110 °C. Thus there is no chance for condensation with water loss. See [Wikipedia — Hypophosphorous acid](https://en.wikipedia.org/wiki/Hypophosphorous_acid).
2. Each monomer has one protic hydrogen. Condensing two of them with loss of water would mean both of the protic hydrogens area in the water instead of the dimer. So if a dimer were to form, it would not be a (Brønsted–Lowry) acid.


**Phosphorus(III): more successful**


With phosphorous/phosphonic acid, formation of the dimeric pyro acid can be accomplished by careful heating. Condensation to form the dimeric acid $\ce{H2(H2P2O5)}$ and possibly higher polymers were achieved at 120 °C, but disproportionation again to phosphine and phosphoric acid occurred at higher temperature. Several phosphite/phosphonate salts also gave generally impure dimeric pyro-salts upon heating.


**Reference**


1. D. Grant, D.S. Payne, S. Skledar (1964).
"The pyrolysis of inorganic phosphites".
*Journal of Inorganic and Nuclear Chemistry*, **26**, 12, 2103-2111.
ISSN 0022-1902,
<https://doi.org/10.1016/0022-1902(64)80155-X>.


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