Sciencemadness Discussion Board

Acidity of Oxo-acids of P

Claisen - 13-1-2011 at 08:23

I am interested in finding out why is the acidity of orthophosphoric acid less than hypophosphorus acid even though orthophosphoric acid has higher basicity ( more number of repleacable H+ ). I need an answer theoretically and not on the basis of their Ka values.

Thanks in advance.

DJF90 - 13-1-2011 at 08:38

The pKa values depend upon the theory. You might want to look at Pauling's rules and the reasoning behind them. Simple "resonance" really.

Claisen - 13-1-2011 at 09:04

Well if resonance explains it then its acidity should be higher again. If all the protons are given, the charge on oxygens can be extensively delocalised through resonance (othophosphoric acid referred).

Btw how pka values are based theoretically? We calculate them experimentally.

[Edited on 13-1-2011 by Claisen]

DJF90 - 13-1-2011 at 09:51

We're considering the pKa1 (2.2) of H3PO4 vs the pKa (2.0 - both from bordwell pka tables) of H3PO2 right? I should have looked up the values before I posted earlier. As you can see, the difference between the two is actually pretty small. If you're considering "which is the stronger acid" then you only need to worry about the first proton exchange as it drops of by about 5 pKa units for each subsequent deprotonation. Both give the same approximate of 3 using Paulings rules, which are suprisingly close in most instances. I'm not sure why H3PO2 is slightly more acidic than H3PO4, but it would be sufficient to say "they're about the same acidity".


I know what you're saying about calculation of pKa values. They're measured experimentally. However, chemistry is an empirical science and so you can look at what factors make a compound a better acid. You can then use this data to be able to predict the pKa of various acids. For example, consider which of 1-propanethiol and 1-propanol are more acidic. Sure, you could measure it in the lab, or even look it up in a pKa table, but being able to predict how factors affect the pKa and thus being able to make an educated guess of the pKa is a useful skill to have.

Claisen - 13-1-2011 at 10:07

Quote: Originally posted by DJF90  
We're considering the pKa1 (2.2) of H3PO4 vs the pKa (2.0 - both from bordwell pka tables) of H3PO2 right? I should have looked up the values before I posted earlier. As you can see, the difference between the two is actually pretty small. If you're considering "which is the stronger acid" then you only need to worry about the first proton exchange as it drops of by about 5 pKa units for each subsequent deprotonation. Both give the same approximate of 3 using Paulings rules, which are suprisingly close in most instances. I'm not sure why H3PO2 is slightly more acidic than H3PO4, but it would be sufficient to say "they're about the same acidity".


hmm.....
Actually this was a question from my course.
Arrange the following acids in the increasing order of acidity-
H3PO3, H3PO2, H3PO4

Here there must be some strong explanation based on which we can arrange them. Lets see if someone else comes up with a clear answer.


smuv - 13-1-2011 at 10:12

[removed]

*stupid misinformed post.

[Edited on 1-14-2011 by smuv]

kmno4 - 13-1-2011 at 14:41

Everything will be clear if you remember that H3PO3 is (OH)2PHO and H3PO2 is (OH)PH2O - then you will get correct order according to "rule": number of hydroxyl groups to non-hydroxyl O - the larger then less acidic.

btw. from physical measurements in gas phase, monomeric HPO3 seems to be very strong acid, much stronger than HNO3 or HCl.


[Edited on 14-1-2011 by kmno4]

smuv - 13-1-2011 at 19:55

Oh great kmno4, that makes sense. I feel better now :)

DJF90 - 14-1-2011 at 04:56

kmno4 - where did you find this rule? I've not seen it before

Claisen - 14-1-2011 at 05:26

Quote: Originally posted by kmno4  
Everything will be clear if you remember


Thats the problem. I am not so good at remembering things without knowing the reason behind them ;)
Can you prove that rule?

kmno4 - 14-1-2011 at 05:52

This is empirical rule and cannot be proved. Surely it has exceptions and limitations, but works for large amount of oxo-acids.
You can predict that H2SO4 is stronger than H2SO3 etc.....
I am not sure but I saw this in Paulings' book (Peter and Linus).
Or it was something very similar.....
However you can call it "Pauling-kmno4 rule".
I will be honoured :D

TheOrbit - 14-1-2011 at 06:12

read this

http://books.google.com.eg/books?id=L-1K9HmKmUUC&pg=PA32...

[Edited on 14-1-2011 by TheOrbit]

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Claisen - 14-1-2011 at 07:19

I read that and got more confused :(
It is written that the acid constants of H3PO3 and H3PO2 are 0.016 and 0.01 respectively, which means that H3PO3 is more stronger (which is against the correct order).

The author is trying to explain why the Ka's of the above two acids differ by only a small amount contrary to the rule 1. I don't see any explanation of why one acid is stronger than the other.

[Edited on 14-1-2011 by Claisen]

[Edited on 14-1-2011 by Claisen]

DJF90 - 14-1-2011 at 08:49

Paulings rules as I am familiar with them state that for an oxo acid (O)aE(OH)b, where a and b are integers, and E represents an element (eg for sulfuric acid a=2, b=2), then the pKa = 8-5a. For each subsequent deprotonation (e.g. for deprotonation of HSO4-), we have to add 5 pKa units. Thus for sulfuric acid, pKa1 = -2, and pKa2 = 3, which is reasonably close for an empirical rule (typically +/-1).

This however is of little use here because we're trying to discern two fairly similarly acidic compounds which both have the same number of oxo groups (i.e. Paulings rules hold, but will not tell us why there is a slight difference). What kmno4 says about the ratio of hydroxyl to oxo sounds reasonable, but I can't see why* it works.