OXIDATION AND REDUCTION 



greatly in its disfavor. The main point is that reaction (2) does not, 

 as a rule, proceed to completion, but that the free radical really does 

 exist, and often even to such an extent that the dimerization is negli- 

 gibly small. There are two powerful tools to decide whether the 

 intermediate product is S, D, or an equilibrium mixture of the two, 

 viz-, potentiometric titration and magnetic measurement. 



When a substance such as duroquinone, the example discussed 

 above, is titrated with a reducing agent and the oxidation-reduction 

 potential is plotted against per cent reduction, a curve is obtained the 

 shape of which will depend markedly on whether the intermediate 

 substance is R or D. Straightforward application of the law of mass 

 action yields a complete theory as to the shape of the titration curve. 

 The calculations involved, although not absolutely simple, are of such 

 a nature that only a high-school student might consider them in the 

 realm of "higher" mathematics. Application of the law of mass action 

 has shown in many cases that the intermediate substance is a free 

 radical, almost exclusively under certain conditions and, under other 

 conditions, in equilibrium with its dimer. Furthermore, according 

 to a theory first developed by G. N. Lewis and later amply confirmed 

 by the quantum theory, a free radical, because it always contains an 

 odd number of electrons, must always be paramagnetic, in contrast 

 to the ordinary, valence-saturated organic compounds, which are 

 diamagnetic (provided they do not contain metal atoms such as iron 

 or cobalt). When a solution of duroquinone is slowly reduced in 

 alkaline solution, by glucose, say, evidence can be produced for the 

 gradual appearance of a paramagnetic molecular species and, on 

 further reduction, for its disappearance. The paramagnetism is due 

 to the spin of the odd electron, while in ordinary molecules the elec- 

 trons always occur in pairs with opposite spin, whereby their para- 

 magnetic effect is quenched. 



An elegant method of detecting such free radicals, even of low 

 stability, has been established by G. N. Lewis. In the kind of experi- 

 ments mentioned above, ejection of an electron is brought about by 

 an oxidizing chemical, which serves as an acceptor of electrons. Lewis 

 eflfects ejection of the electron by ultraviolet light, the substance being 

 dissolved at the temperature of liquid air, in an organic solvent which 

 has, at this temperature, a rigid, glasslike consistency without crystal- 

 lizing. In such a rigid medium, no molecular collisions can occur 



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