LEONOR MICHAELIS 



occurs. We have presented the postulate that all oxidations proceed 

 in a sequence of univalent steps. The first step of oxidation of alcohol 

 would lead to the free radical, in this case, an utterly unstable molecule, 

 in the thermodynamic sense. To generate these radicals, an oxidizing 

 agent of very high potential is required ; and even then their concen- 

 tration remains small, so small that no direct evidence for their exist- 

 ence is available. The free radicals, once generated, will then react 

 by a dismutation: 



2 radicals ^ ^ 1 alcohol -}- 1 aldehyde 



The velocity of the latter reaction depends, among other things, on 

 the concentration of the molecules which are interacting with each 

 other, and therefore on the square of the concentration of the free 

 radicals. If this concentration is very small, it may be the limiting 

 factor for the over-all process of oxidation of alcohol. We may say 

 that the energy of activation for the oxidation of alcohol is essentially 

 the energy necessary for the formation of the free radical. Unless 

 the radical is relatively stable, as in reversible processes, the activation 

 energy is very great. This high energy of activation is the reason why 

 so many organic compounds are "stable." If all thermodynamically 

 possible reactions could proceed unhampered there would be no such 

 thing as organic chemistry. 



Inhibition due to high activation energy occurs only when the 

 attainable concentration of the free radical is the limiting factor for 

 the rate of the over-all reaction. It does not matter whether the 

 attainable concentration of the free radical is 1 Af or, say, 10~* M. 

 Factors other than the concentration of the free radical, such as the 

 specific constants of reaction velocities, will determine the rate of the 

 reaction. However, if the concentration of the free radical is so small 

 as to be the limiting factor of the over-all process, sluggishness and 

 irreversibility will arise. This consideration fulfills an important re- 

 quirement for the understanding of reaction rates — it reduces a problem 

 of kinetics to one of thermodynamics. 



Oxygen as an Oxidizing Agent 



These considerations also explain why oxygen is such a sluggish 

 oxidizing agent despite its very large oxidative power in a thermo- 



220 



