OXIDATION AND REDUCTION 



tetravalent, oxygen bivalent, and iiydrogcn univalent: the uni\alenl 

 oxidation of CH4 would give, as a step intermediary on the way to 

 CH3OH, the free radical CH3 with "tervalent" carbon. To be sure, 

 such free radicals have been recognized for a long time, but those 

 known until recently always have one of two properties: either they 

 are very unstable and have an extremely short lifetime (e. g., CH3); 

 or they can be produced only from a very restricted number of com- 

 pounds and only in a solution of a perfectly water-free organic solvent. 

 The famous discovery of triphenylmethyl by Gomberg in 1890 pro- 

 vided a prototype of such free radicals. 



It will now be shown that all oxidations of organic molecules, 

 although they are bivalent, proceed in two successive univalent steps, 

 the intermediate state being a free radical, and furthermore, that 

 according to structural conditions these intermediate free radicals may 

 be either just as unstable as CH3, somewhat more stable, or even 

 perfectly stable compounds. 



First, however, the concept of stability must be discussed. The 

 criterion of stability may be obtained in two ways. A substance may 

 be said to be unstable if it rapidly undergoes a chemical change when 

 exposed to such ubiquitous reagents as air; pyrogallol, for instance, is 

 unstable in an alkaline solution because it is readily oxidized by 

 oxygen, but is stable in the absence of oxygen. Or, a substance may 

 be said to be unstable if it undergoes a change even in the absence of 

 any foreign substance with which it might react; acetaldehyde, for 

 instance, undergoes the Cannizzaro reaction in an alkaline solution, 

 one molecule of aldehyde being oxidized to form acetic acid, while 

 another is reduced to ethyl alcohol. It will now be shown that the 

 alleged instability of free organic radicals is, in very many cases, 

 essentially due to the latter, bimolecular interaction. It will be 

 shown that the bivalent oxidation in fad occurs in two successive 

 univalent steps as in the example of duroquinone (X). 



The methyl groups in the ring of X prevent the secondary, 

 irreversible reactions which are of no interest in this discussion, and 

 which would occur when working with the unmethylated, simple 

 benzoquinone. When duroquinone in an alkaline solution is reduced, 

 as by hydrogen plus palladium, or by any other suitable reducing 

 agent, the faintly yellow solution turns brown first, then colorless. 

 The colorless compound is the corresponding durohydroquinone, which 



213 



