Is the effect of hydroperoxyl radicals significant in the oxidation of organic compounds in the Fenton reaction? Why or why not?

Question

In the Fenton reaction, a ferrous catalyst and hydrogen peroxide are used to generate hydroxyl radicals which in turn oxidise organic compounds. The reaction is commonly seen in the treatment of wastewater and operates using the following mechanism:

$$ \begin{align} \ce{Fe^2+ + H2O2 &-> Fe^3+ + ^.OH + OH-}\tag{1}\\ \ce{Fe^3+ + H2O2 &-> Fe^2+ + ^.OOH + H+}\tag{2}\\ \ce{Fe^3+ + ^.OOH &-> Fe^2+ + H+ + O2}\tag{3}\\ \ce{RH + ^.OH &-> H2O + R^{.}}\quad\text{(chain propagation)}\tag{4}\\ \ce{R^. + Fe^3+ &-> R+ + Fe^2+}\tag{5}\\ \ce{R^. + H2O2 &-> ROH + ^.OH}\tag{6} \end{align} $$

In the reactions, hydroperoxyl radicals $(\ce{^.OOH})$ are produced as a byproduct of the reduction of the ferrous catalyst, among other reactions. However, literature regarding the impact of the hydroperoxyl radical on the oxidation of organic compounds is limited, with most papers ignoring the effect of the hydroperoxyl radical completely. So, is the effect of the hydroperoxyl radical significant in the oxidation of organic compounds? Why or why not?

Answer 1

The effect of the hydroperoxyl radical in the Fenton process can be largely ignored as the oxidising ability of the $\ce{^∙OOH}$ radical is not as effective as that of the hydroxyl radical.

According to Chang et al., the reaction rate coefficients of p-nitrophenol and $\ce{OH}$ and $\ce{HO2}$ were found to be $\pu{9.05 \times 10^6 M−1 s−1}$ and $\pu{1.50 \times 10^−6 M−1 s−1}$, respectively. Considering the orders of magnitude of these two rate coefficients, the reaction pathway involved the p-nitrophenol reaction with $\ce{^∙OOH}$ radicals can thus be neglected. This can therefore be extended to cover the oxidation of organic compounds by hydroperoxyl radicals through the Fenton process.

References

1. Chang, M., & Chern, J. (2010). Decolorization of peach red azo dye, HF6 by Fenton reaction: Initial rate analysis. Journal of the Taiwan Institute of Chemical Engineers, 41(2), 221-228. DOI: 10.1016/j.jtice.2009.08.009