[Glorzifen]

Hi,

Could someone explain what 'Oxyacid Resonance Stabilization' refers to? I'm looking at a comparison between sulfurous acid and sulfuric acid. Sulfuric acid has more oxygens, and so the electron density is attracted further away from the hydrogen (when compared to the 3 oxygen of sulfurous acid)...which means sulfuric acid is a stronger acid.

I'm not seeing the connection between what being a stronger acid has to do with more resonance structures. Is it just because it has more oxygens, there are more combinations (resonance structures) possible?

Thanks,

G

[Dan]

This is a bit of a generalisation, but the more resonance structures you have, the more delocalised the charge, and therefore the more stable the species.

Take for example the following conjugate bases:

SO₄^2- - I can draw 6 equivalent resonance structures
SO₃^2- - I can draw 3 equivalent resonance structures

We would expect the negative charge to be more delocalised for the sulfate anion, and therefore sulfate to be a more stable conjugate base. The more stable a conjugate base is, the more acidic the acid it came from is (ie. conjugate base forms more readily for for a bisulfate than a bisulfite).

This does turn out to be the case, sodium bisulfate (NaHSO₄) has a pKa of about 2, and sodium bisulfite (NaHSO₃) closer to 7 (this is a difference of 5 orders of magnitude!)

[Glorzifen]

Thanks.

Just to clarify what you mean be 'delocalised charge', it seems that for the sulfate anion...the electron density would be greater around the oxygens than around the sulphur atom; is that correct? If so, is that what makes it more stable? I would think that being more stable means that the electron density is more evenly distributed throughout the structure of the molecule, but could be wrong.

I think I'm just not understanding what is meant (in terms of electron density or other specifics) by delocalised charge.

[Dan]

So when I say the charge is delocalised, I mean it is spread over the oxygens rather than being stuck (localised) on one oxygen. The more atoms the charge is spread over the better - and in the case of oxyacid conjugate bases it is the oxygen atoms that carry the charge (as you have noted).

In the case of sulfate the 2- charge is spread (delocalised) over 4 O atoms, in the case of sulfite the 2- charge is spread over 3 O atoms. So, there is more delocalisation in the case of sulfate than sulfite, meaning sulfate is more stable than sulfite, and consequently the bisulfate ion is more acidic than the bisulfite.