Chemistry Why is the third hydrogen attached to P instead of O in phosphorus acid?

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The discussion centers on why phosphorus acid (H3PO3) has one hydrogen attached to phosphorus instead of all three hydrogens attaching to oxygen. Participants note that the stability of the P(OH)3 form is questionable, and its instability suggests that the observed structure is energetically optimal. The conversation highlights that while some molecular structures can be explained through generalized rules, unique cases like phosphorus acid may not have straightforward explanations. The equilibrium between different forms of phosphorus acid is mentioned, with a significant preference for the observed structure. Ultimately, the lack of a simple explanation reflects the complexity of molecular stability in inorganic chemistry.
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Stumbled upon this when I was looking at lewis dot electron structures. The phosphorus acid is dibasic. What prevents the third hydrogen from attaching to the third oxygen instead of the central phosphorus? Is there a theoretical explanation or we can only memorize this for a fact?

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Wouldn't that leave two empty phosphorus bonds? You wouldn't like phosphorus when it's angry.
 
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P will have 2 non-bonding electrons, its formal charge will still be 0. Things seem okay. What do you mean by two *empty* bonds?
 
So you mean why P(OH)3 is instable? Not sure, but similar instabilities apply to e.g. orthocarbonic acid C(OH)4 which dehydratizes to H2CO3. The esters P(OR)3 are stable, but can convert to O=PR(OR)2.
 
No, that's not the question. The question is why in H3PO3, two Hs are attached to O, but one O is attached to P instead of all 3 Hs attached to all 3 Os.
 
guv said:
No, that's not the question. The question is why in H3PO3, two Hs are attached to O, but one O is attached to P instead of all 3 Hs attached to all 3 Os.

@DrDu is on point - you are asking why P(OH)3 is unstable. Were it stable it would be the form observed - but it isn't.

Which sometimes means "when you calculate the energies turns out the one we observe is optimal" - and there is not other explanation than "because that's the way it is". We can try to find some generalized "why?"(and it often nicely works as a rule of thumb for things that tend to repeat in many observed molecules), but the more rare the case is, the less valuable these ideas are - they don't add any real insight.
 
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Ah I see, that was my question. I was expecting a hand waving explanation instead of just how it is what it is. It doesn't sound like there is a hand waving explanation in this case.
 
Turns out that wikipedia contains some information on the tautomeric equilibrium between phosphonic and phosphorous acid: https://en.wikipedia.org/wiki/Phosphorous_acid
So both forms P(OH)3 and HPO(OH)2 exist but the equilibrium constant is K=10^10.3
For homologous arsenic, As(OH)3 is more stable. I am not sufficiently into inorganic chemistry to be able to provide a hand-waving explanation although it is certainly possible to come up with one knowing the size and energies of the orbitals etc.
 
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