OXIDATION AND REDUCTION 



and the hydroquinone. If we designate the hydroquinone (the re- 

 duced form of this system) as R, the semioxidized form (the free radical) 

 as S, and the totally oxidized form (the quinone) by T, the equilibrium 

 is established according to the reversible reaction 



2S , ' R + T (1) 



and the constant of equilibrium, which may be called the "semi- 

 quinone formation constant," is: 



_[S?_ 

 [R][T] 



Its reciprocal may be called the "dismutation" constant, because re- 

 action (1) is a "dismutation" (or "disproportionation") of the free 

 radical. If k is very small, very little of the radical exists in the state 

 of equilibrium; if k is large, much of the radical can exist, and it can 

 be distinguished by its particular color and by its paramagnetism, 

 which will be discussed presently. 



Experience has shown for the case of duroquinone that k is very 

 large in an alkaline solution, but very small in an acid solution. The 

 transition of the behavior from alkaline to acid solution is continuous; 

 and there can be no doubt that in acid solution a small amount of the 

 radical is formed too, even if its concentration is too small to be notice- 

 able by ordinary methods. Thus we are obliged to state that the 

 ionized form of the radical (XIII) is a rather stable compound, and 

 the unionized (XIV) rather unstable, "stability" being judged accord- 

 ing to its capability of existing in equilibrium with its parent substances, 

 the quinone and the hydroquinone. 



Why is XIII more stable than XIV? In formula XIII, the 

 negative charge has been arbitrarily attributed to the upper oxygen 

 atom, but it can just as well be attributed to the lower one. In fact, 

 the location of the charge is undecided, for it oscillates between the 

 two extreme positions through the chain of the atoms in the ring. 

 Such a condition has been termed resonance. The two limiting struc- 

 tures, one with the charge on top, the other with the charge at the 

 bottom, are indistinguishable molecular species. One speaks about 

 "equivalent" or "symmetrical" resonance, a condition which, accord- 

 ing to quantum mechanics, contributes largely to the stability of a 

 molecule. In form XIV, no such equivalent resonance prevails. The 



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