LEONOR MICHAELIS 



in extremely alkaline solution can be written as formula XI and which 



O -O OH 



i k 



CH3— /N— CHs 



CH3 



\ 



-CHs 



— CH3 



— CH3 



-o 



XI 



in less alkaline solution would add first one, then another, proton to 

 form XII. This is the customary bivalent reduction. The inter- 

 mediate, brown substance is the result of a univalent reduction; and 

 it can be shown by various methods, to be described later on (see 

 pages 215 and 217), that it has the same molecular size as XI and 

 differs from it only in so far as one oxygen atom is negatively 

 charged and not the other. This brown substance is a free radical. 

 One might say that one oxygen atom is bivalent and the other uni- 

 valent, as in XIII. The same molecular species, in a more acid solu- 

 tion, would have formula XIV. 



O O 



GHj 

 CH, 



^. 



-CH3 

 -CH3 



CH3 

 CH3 



,-V^-CH3 

 -GHs 



o- 



XIII 



OH 



XIV 



Let us consider first the reaction in a strongly alkaline solution. 

 When the solution of the quinone is mixed with increasing amounts 

 of a reducing agent, there will always be, except for the very begin- 

 ning and the end of the oxidation, a mixture of the quinone, the hydro- 

 quinone, and the intermediate free radical which we shall call semi- 

 quinone; and an equilibrium is established between these forms. It 

 is important to emphasize that the equilibrium is established instan- 

 taneously, and that there is no sluggishness in its formation, as is usually 

 the case in the formation of equilibria with organic compounds (except 

 acidic ionizations). The activation energy of this reaction leading to 

 equilibrium is extremely small. So, in the solution, the intermediate 

 free radical is never present without being in mixture with the quinone 



214 



