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.
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$\begingroup$ Look at the structure of the acid attentively, it should become obvious. Hint: the way you wrote it is misleading. $\endgroup$– permeakraMay 21 at 14:16
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1$\begingroup$ So what I could understand is that since H3PO2 is a monoprotic acid if we took two molecules of H3PO2 and removed two molecules of water, we would lose both the acidic hydrogen. Hence, we won't have an acid at all so no pyroacid either. correct me if I am wrong. $\endgroup$– Mathlover12345May 21 at 14:37
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2$\begingroup$ $\ce{H2(PO)-O-(PO)H2}$ is not an acid, like PH3 is not an acid. More exactly they are so weak acids we can forget about that. For similar reasons, there are no dinitrates, diperchlorates etc. $\endgroup$– PoutnikMay 21 at 15:51
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2$\begingroup$ Browsing through Google and Wikipedia, it seems that $\ce{H3PO2}$ is sensitive to heat. When heated at $100$°C, it is disproportioned through :$$\ce{2 H3PO2 -> PH3 + H3PO4}$$ No pyro acids has the time to be formed. $\endgroup$– MauriceMay 21 at 16:01
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$\begingroup$ I have forgotten to use the third conditional in the first comment. $\endgroup$– PoutnikMay 22 at 8:40
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1 Answer
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It’s essentially both reasons mentioned in the comments.
“Pyro-” means “fire” or, more generally, “heat” (“pyrotechnics” = “fireworks”). 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.
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
- 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.
