LEONOR MICHAELIS 



proton attached holds the negative charge rather tightly in place. 

 This molecule does exist but to a much smaller extent than XIII be- 

 cause it is less stable. Free radical formation is a general reaction 

 whether it takes place extensively, as in alkaline solution, or in traces, 

 as in acid solution. 



It can be shown by numerous examples that the same behavior 

 is true for organic compounds which form reversible oxidation-reduc- 

 tion systems, such as many dyestuffs, e. g., methylene blue, or phenol- 

 indophenol. No observable amount of any intermediate free radical 

 can be demonstrated for irreversible oxidations, e. g., when alcohol is 

 oxidized to aldehyde. So we are permitted to conclude that the 

 formation of a free radical, in sufficient concentration, in the equi- 

 librium involved, is a prerequisite for the reversibility of an oxidation- 

 reduction system. 



Evidence for Free Radical Nature 

 of the Intermediate Substance 



It is appropriate to discuss at this point the evidence for the 

 assertion that the intermediate steps of oxidation are really free radicals 

 of the same molecular size as the parent substance and not dimeric 

 valence-saturated compounds made up in such a way that two radicals 

 are combined to form a bond which abolishes the state of "unsatura- 

 tion." Such a reaction may be imagined to occur as follows: 



2 S > D (2) 



That is, two molecules of the semiquinone radical, S, combine with 

 each other to form a dimeric compound, D, which no longer has the 

 characteristics of a free radical, just as two "radicals" H (hydrogen 

 atoms) combine as follows: 



2.H > H2 (3) 



In fact, such reactions do occur, and an equilibrium is established, so 

 that, instead of reaction (2) we should write: 



2 S , D (4) 



Depending upon the conditions, which will not be discussed here, this 

 equilibrium is sometimes greatly in favor of the free radical, sometimes 



216 



