The pKa1 of phosphoric acid is 2.14. As far as I am concerned, phosphoric acid does not have any double bounds and, as a main-group element, obliges to the octet-rule. This does create formal charges but these are preferred over breaking the octet-rule. This is further explained in this article.
This begs the question though: How can one explain the low pKa.
Resonance seems to be off the table due to the fact that there cannot be any resonance if there is no conjucated system.
As far as I am concerned, expanded octets are incorrect and cannot be applied here - if they can even be applied anywhere since I believe this theory is somewhat outdated.
Inductive effects seem like they could be of use here since there are indeed 3 OH-groups and one O (the one with the negative-formal charge). But I believe that this should not be able to explain this pKa for two reasons.
First of all, all of these groups should theoretically draw electron-density to themselves. This would not make the bond between the H and O more unstable and thus would not result in a higher chance of dissosiation of the proton.
Furthermore, when comparing the pKa from the first protolysis of phosphoric acid to the one from ethanol (pKa=15.9) the differnece seems to be far too great to be due to an inductive effect.
Continuing this thought though, I have had the idea that maybe this relativly strong pull of electon density - due to the number of groups with a negative inductive effect -could stabilise a negative charge, thus making the first protolysis more appealing. A similar thought, but this time with the polarizability of the phopshorusatom in mind, is explored in the following.
Assume one or more protons have already been taken away, leaving behind a negative charge. The elektostatic repulsion of the bonding elektrons (and maybe others aswell) from this negative charge would push the electron-density towards the comparibly high polarisable phosphorusatom. This in turn could lead to a somewhat stabilised negative charge. Now I know that there is no "real" delocalisation to apply here - this is just a thought I have had.
I have also thought about something similar to the erlenmyer rule. My theory was that due to the multiple OH-groups on the same phosphorus there might be a similar reactivity as with carbonatoms with multiple OH-groups. But as far as the feedback I have recieved from discussing this posibility, this seems like a far fetched assumption and I have to agree.
The last idea has something to do with constitutional isomers (more specific:tautomers). H-Atoms are know for their behaviour to swap places. If, for example, one proton has already been taken away, the remaining H-Atoms could swap their place to the negative charge because of the electrostatic attraction. This would lead to a somewhat delocalised proton (or multiple protons) leaving the the negative charge to be also delocalised making it harder for it to accept a different proton (for example from water). (I believe the term "delocalisation" is misplaced here since there is no conjucated system and thus no resonance but I could not find a better way to describe it)
As you can see I am very confused but intrigued by this topic. Most of the sources I have looked at and professors + colleagues I have talked to, draw the phosphoric acid with a double-bounded O-Atom (oxo) and explain its pKa with resonance. This makes it incredibly frustrating to figure anything out about this. This is why I am searching for help here and sharing all of my thoughts - no matter how wrong some of them sound. I hope somebody can help me - and others with a similar question -out.
Thank you
