# Why is the OO-H dissociation is unlikely for the Caro's Acid?

Let us consider the Caro's Acid: $$\mathrm{H_2SO_5 \equiv SO_3H-OOH}$$. It is widely known that the hydrogen tied with the $$\mathrm{-OO-}$$ group is pretty much unlikely to dissociate: $$\text {Terminal dissociation: }\mathrm{SO_3H - OOH \rightleftharpoons H^+ + [SO_3-OOH]^-} \\ \text{An unlikely step: }\mathrm{[SO_3-OOH]^- \rightleftharpoons H^+ +[SO_3-OO]^{2-}}$$ The first time I encountered that fact I was presented a pretty inconclusive piece of argumentation: it was stated that $$\mathrm{-OO-}$$ group is way more electronegative than $$\mathrm{-O-}$$, which somehow brings about an obstacle for the dissociation.

Unfortunately, it was a long time ago and I cannot remember the exact details. I stumbled upon this problem not so long ago. Having a better background in chemistry, I attempted to apply the Sanders' electronegativity balance theory, but the result was pretty inane in terms of our chemical reality: $$\chi_{eq}(\mathrm{H_2SO_4}) \approx 2.89894; \chi_{eq}(\mathrm{H_2SO_5}) \approx 2.96803$$. In other words: $$\chi_{eq}(\mathrm{H_2SO_5}) > \chi_{eq}(\mathrm{H_2SO_4})$$.

What am I doing wrong? Is it possible to tweak my attempt to use Sanders' theory? Is there any other approach to this problem?

• It's just not delocalised there, that's it about it. – Mithoron Apr 11 at 18:51
• @Mithoron well, that’s a good point, but the localization analysis is, in fact, just another way to look at electronegativity computational models. I mean, the resulting electronegativity does depend on the electron-nucleus relations (i.e. ionization and electron affinity energy). So, to be honest, this does not really answer my question. I may have worded it a bit clumsily: I’m interested in the computational aspect of the problem (and the application of the Sanders’ theory, to be more precise). – Zhiltsoff Igor Apr 11 at 21:06
• So the question: why is it a peroxide instead of e.g. an oxide? – Greg May 17 at 2:17
• Hmmm... What is the "Sanders' electronegativity balance theory" you are referring to? I believe most of us in the community are not well-versed with this theory. Perhaps, you could provide a reference for us to look at so that we can learn about this theory? – Tan Yong Boon May 17 at 12:32
• Based on this reference (dash.harvard.edu/bitstream/handle/1/11913975/…), the first $\ce {pK_a}$ of the acid is 0.4, which is higher than that of sulfuric acid's corresponding first $\ce {pK_a}$ value of around -3. It is not surprising due to the fact that there is a greater extent of delocalisation of the negative charge in the sulfate conjugate base compared to in the peroxymonosulfate conjugate base. Note that the 2 $\ce {O}$'s in the peroxy group are not able to participate in resonance. – Tan Yong Boon May 17 at 12:51