[maakii]

Hi everyone,

I would like to know which of the phosphate acids is most acidic, taking into account only the first dissociation

Phosphoric acid, H₃PO₄
Phosphorous acid, H₃PO₃
Phosphinic acid, H₃PO₂

I was guessing that phosphoric acid would be the most acidic, as it has the most oxygen atoms to draw electron density from the H₂PO₄^- anion, but it seems this is wrong. Can anyone tell me why? Thanks (;

[Bakegaku]

Your reasoning is right, but your wording is wrong.  You want to take electron density away from the H⁺ ion(s) to make an acid stronger.

[maakii]

Oh I see, that makes sense too..

But on wikipedia pKa1 of phosphoric acid is 2.12, for phosphorous acid its 2, and for phosphinic acid its 1.2. But if more oxygen atoms makes it more acidic, this trend is not reflected in the real pKa values, so I'm quite confused!

[english]

Acidity of phosphorus oxoacids is affected by (1) probability of hydrogen removal and (2) like-like charge repulsion.

(1) The more H you remove, the less H there are to remove.

(2) The more H you remove, the higher the net charge on the conjugate base, and thus the harder it becomes to remove an acidic H.

[maakii]

Umm, I'm sorry if I have confused you too,

But I am comparing the 1st pKas of different phosphorous oxoacids(H3PO4, H3PO3, H3PO2), and not the 2nd and subsequent pKas. They are all neutral to begin with

[english]

Oh,

Well the most important effect is number of bonded O.

[maakii]

Yup that's what I thought, but phosphinic acid, with the least no. of bonded oxygens, is most acidic.

Probably I should include some pictures here too

Phosphoric acid
Phosphorous acid

Phosphinic acid

[enahs]

Acids of a central atom surrounded by O atoms and OH groups are termed Oxy Acids.

They can be represented as
XO_n(OH)_m

The magnitude of K, in general, depends on the value of n, the number of additional oxygen atoms other then those in the OH group; which is where you are making your mistake.

In all three acids, they have only 1 Oxygen atom not in a OH group surrounding the central P; as a result they all have the same general acid strength.



I guess an easy way to explain it would use inductive effect, though a MO approach would be better.

Look at Phosphoric acid. The =O bond is highly polar, but it is pulling against 3 equivalent OH groups, as a result the 3 equivalent OH groups experience very little inductive effect, and as a result, on a whole the molecule is not very polar with respect to the OH's.

If we look at Phosphorus acid, the =O bond has to only pull on two OH groups, as a result, in general those two OH groups experience more of an inductive effect, and as a result those two Oh groups are slightly more polar the Phosphoric Acid.


Hypophsophorus acid the =O can apply all its "inductive effecteviness" on only one OH group, making it much more polar and easier to remove.


Now, to get a grasp on the inductive effect theory, look at carboxylic acids:
http://en.wikipedia.org/wiki/Inductive_effect


Now, you might then want to ask why then is H atoms attached directly to the P in Phosphorus and Hypophosorus acid not the H to be the acidic acid?
You can sorta justify this with a resonance approach:
http://en.wikipedia.org/wiki/Resonance_(chemistry)

But, trying to use these two methods to understand it gets very convoluted, and starts causing all sorta of trouble.

The three you, or your teacher happened to pick, just so happen to be the exceptions to the Oxy Acid rule, and to explain it with any real kind of merit requires a Molecular Orbital approach (and a very complicated on at that, complicated relative to why you might learn O₂ is paramagnetic using MO).


Now, it is late, maybe I am just missing something super simple...

[maakii]

Hey I think I understand this now, but one thing I would like to clarify is as you said, the =O group is highly electron withdrawing, but isn't the -OH group electron withdrawing as well?

If we look at phosphoric acid, can we not imagine both the =O group and 2 -OH groups to draw electron density from the 3rd OH group? Or is it because the effect of the OH groups on other OH groups is 0 since they fight for electron density equally? I would guess that this is the case based on your explanation..

Thanks for helping