CHEMICAL PROPERTIES 431 



diate free radical semiquinone, the stability of which depends mostly on 

 the nature of the quinone and the pH, and may be such that the semiqui- 

 none can exist in appreciable concentrations (Michaelis, 1935). Some have 

 believed that the semiquinone is the active form in producing certain effects 

 on enzymes, and in view of the particular reactivities of such free radicals 

 it is necessary to take them into account. Semiquinones may (1) be oxidized 

 to the quinone, (2) reduced to the hydroquinone, (3) dimerize to form quin- 

 hydrone, or (4) react with various substances present: 



^ QH- ^ > QH, 



Reactions 



and thus the concentration of semiquinone in any system will depend on 

 many factors. The tetrasubstituted semiquinones, such as durosemiqui- 

 none, are particularly stable, since the groups sterically hinder dimeriza- 

 tion and promote resonance between the semiquinone anions (Wheland, 

 1955, p. 388). The absorption spectrum of the semiquinone sometimes dif- 

 fers sufficiently from those of the quinone and the hydroquinone so that 

 the semiquinone can be detected photometrically. The semiquinones are, 

 of course, paramagnetic, and this has been used to detect the relatively 

 stable Na+ salts of several tetrasubstituted semiquinones (Kainer et at., 

 1956). The hyperfine interaction between the unpaired electron in certain 

 semiquinones with protons has been detected by paramagnetic resonance 

 spectroscopy (Venkataraman and Fraenkel, 1955) and p-benzosemiquinone 

 finally observed (Blois, 1955). The spectroscopic splitting factor, or g value, 

 is given for several semiquinones by Blois et at. (1961). The stability of the 

 semiquinones increases with increase in the pH since resonance is favored 

 in the anionic forms. The free energy changes for each of the two steps in 

 the oxidation of a hydroquinone are different. The total 33 kcal/mole in- 

 volved in the oxidation of p-benzohydroquinone could be divided by Fieser 

 (1930 b) as follows: 



QH2 ±? QH • + H • AF = 22.5 kcal/mole 



QH-^Q +H- AF = 10.5 kcal/mole 



From the equilibrium constants for the two reactions, it was calculated 

 that the concentration of semiquinone in a solution containing 100 mM 

 of both 2?-benzoquinone and p-benzohydroquinone would be around 

 0.004 mM. 



