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

[guv]

Homework Statement

As stated in the title

Relevant Equations

N/A

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?

Thanks,

 

[Algr]

Wouldn't that leave two empty phosphorus bonds? You wouldn't like phosphorus when it's angry.

 

[guv]

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?

 

[DrDu]

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.

 

[guv]

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.

 

[Borek]

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.

 

[guv]

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.

 

[DrDu]

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.