I thought I was done here, but the xanthine solubilities post
got me thinking and then the additional post by Kran. I have written an additional blog
in which I argue stereoelectronic control virtually precludes a resonance effect.
Re: acetic, peroxyacetic, and resonance effects
I find this data in accord with what one should expect for an inductive effect. For example, hypochlorous acid (HOCl) has a pKa of 7.5 and hydrogen peroxide, also not resonance stabilized, has a pKa of 11.6. These pKa’s are in the range of peroxyacetic acid and could have been expected. I would characterize an acetate as a weaker electron withdrawing group than chloride. The poster also stated that the acidity was reduced in peracetic acid because it was not resonance stabilized. Although I agree that peracetic acid is not resonance stabilized, I disagree that resonance is why acetic acid is a stronger acid. Note the pKa for pyrrole, 17.5. Even though a pair of non-bonded electrons are present on the nitrogen atom, the newly formed electrons of the anion are not participating in the resonance structure. The anion electrons are orthogonal to the pi-electrons, yet the pKa is much lower than a simple amine (~35).
The poster made a second point about bond length and bond strength. Bond strength arguments are frequently made in a rather casual manner. If you were to find bond strength data, it refers to homolytic bond strength. If homolytic bond data were used, then a completely different and incorrect prediction of acidities would result. Acidity is a heterolytic bond cleavage. Secondly, the bond length data does agree with heterolytic bond strengths. Here is why. If you compare the bond lengths of CH4, NH3, H2O, and HF, HF has the shortest bond. It also is the most acidic. The key to understanding acidity is the proton-electron pair distance. We cannot measure that directly, but if the inverse square law applies (and it does), then the greater the proton-electron pair distance, the weaker the bond. Therefore, we know as the electrons are pulled closer to the nucleus, they are pulled away from the proton resulting in a weaker bond. Additionally, the bond length tells us the distance between the proton and oxygen nucleus in this case. The charges of both are the same, therefore the closer the proton is to the nucleus, the greater the repelling force. This repelling force is complimentary an increased proton-electron pair distance and results in the increase in acidity.