Nucleophilic character of HOOH

Because the HO-OH bond in hydrogen peroxide is weak (AHdbE/ 51 versus 90 kcal for the H-OOH bond), there has been a tendency to assume homolytic cleavage in the reactivity and activation of HOOH. However, in a pioneering study of the conversion by HOOH of sulfur dioxide to sulfuric acid Halperin and Taube 2 proposed a nucleophilic-addition mechanism. 

In a subsequent investigation s of this system with anhydrous HOOH in dry MeCN, we discovered that there is no net reaction in the absence of H2O. The reaction is first order with respect H2O up to a 200-mM concentration, and becomes second and third order at higher concentrations. The reactivity of f-BuOOH with SO2 also exhibits a similar first order dependence on H2O concentration, but t-BuOOBu-t is unreactive under all conditions (the f-BuO-OBu-f bond energy is essentially the same as that for the HO-OH bond). Thus, water induces HOOH (pKa, 11.8) to act as a nucleophile towards SO2 (pKg, 1.9) with the reaction rate fastest at pH 1.9. 

If HOOH can act as a nucleophile in aqueous solutions at pH 1.9, then stronger bases in aprotic media [e.g., pyridine in py2(HOAc) and MeCN solvents] should facilitate similar pathways. Hence, the Fenton process in H2O (at pH 2 to 10)8 and in aprotic media [py2(HOAc)] must involve nucleophilic addition 

Reactivity of Hydrogen Peroxide, Alkyl Hydroperoxides, and Peracids (PA)2FeH + HOOH + py [(PA) FeD-OOH + Hpy+] 

Likewise, the activation of HOOH by horseradish peroxidase to Compound I probably involves a similar pathway, for example. 

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