OXIDATION AND REDUCTION 



dynamic sense. If it can be reduced only in successive univalent 

 steps, these steps must be: 



O2 > O2- »02-- >Oj" ^O*" 



Two of these steps are chemically identifiable. O2 is, after accept- 

 ing two protons, O2H2, hydrogen peroxide. ©2" is, after accepting 

 four protons, two molecules of H2O. However ©2" (or O2H), and 

 02~ (which may be written as O2H3, or OH + H2O) are intermediate, 

 utterly unstable steps. Since the reaction must pass through these 

 unstable steps, the activation energy involved in the reduction of O2 

 is very high. 



How is this activation energy overcome when oxygen does 

 oxidize a substance? Overcoming the activation energy by working 

 at high temperatures is a usual procedure in the laboratory but is not 

 feasible under physiological conditions. Here the answer is that 

 oxygen reacts with an oxidizable substance very often not only by 

 means of a collision of the molecules but also by the establishment, 

 after collision, of a relatively stable addition compound which can 

 then undergo intramolecular redistribution of electrons. 



Certainly no claim is made that this mechanism is always the 

 one involved in activation of oxygen. However, it is one of the possible 

 mechanisms and very likely is correct for the particular case to be de- 

 scribed in detail. In all probability it is the inechanism by which 

 oxygen is activated in all those cases in which a heavy metal com- 

 pound, especially of iron or copper, acts as activating catalyst. 



It has been shown that, at least in an acid solution, the oxida- 

 tion of a leuco dye, or of cysteine, and many other substances, by 

 means of free oxygen is accomplished, at least with any appreciable 

 speed, only in the presence of a trace of an iron or copper salt. These 

 metal atoms have two essential properties which render them useful 

 for their catalytic action. First, they readily change their valence; 

 iron may be bivalent or tervalent and copper, univalent or bivalent. 

 Second, these metals are highly inclined to form complex compounds 

 of the Werner type. The nature of such metal complex compounds 

 may be demonstrated as follows. The doubly positively charged 

 ferrous ion, Fe+"^, can combine, first of all, with two negatively charged 

 univalent ions to form a saltlike compound, for instance, with two 

 cyanide ions: 



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