**4.3 Comparison of (Fe = O)+2 and (Fe = O)+3 ion chemistry**

Both (Fe = O)+2 (**Figure 2**) and (Fe = O)+3 ions (**Figure 3**) abstract H• from the weakest C-H bond present in a molecule to form ferric (Fe+3OH) or ferryl hydroxide (Fe+4OH) and a C• radical respectively [85].

*A History of the Fenton Reactions (Fenton Chemistry for Beginners) DOI: http://dx.doi.org/10.5772/intechopen.99846*

The electrophilic ferric and ferryl hydroxides react 'instantaneously' with the nucleophilic C• radical, but the resulting intermediates are different. Ferric hydroxide donates HO• to the C• radical, regenerating the ferrous ion, ending the cycle [33], however the ferryl atom attacks the C• radical (ejecting the hydroxyl group) to form the ferryl-carbon (Fe+4-C) intermediate [83]. Oxygen (O2) insertion into the (Fe+4-C) bond creates the bifurcated oxidative pathways not available to either ferryl-oxo ion or hydroxyl radical [86].

Sugimoto et al. (1987), using <sup>2</sup> H and 18O labeled ethanediols, determined that H• abstraction by (Fe = O)+3 from the hydroxyl oxygen of a diol group [R1-HC(O*H*)-HC (OH)-R2] causes C-C bond cleavage, producing paired aldehydes [R1-HC=O + R2- HC=O], whereas H• abstraction from the carbon backbone produces hydroxy-ketones [R1-C(O)-HC(OH)-R2] [87].

### **5. Mixed Fenton oxidation systems**
