REACTIONS OF THE QUINONES 437 



The forms (I), (II), and (III) are tautomeric isomers and are in equilibrium, 

 but (III) appears to be much more stable than the others, due to the 

 aromatic resonance energy, so that it is the dominant product. The thio- 

 hydroquinone (III) will then usually be oxidized to the quinone, either by 

 unreacted quinone: 



S— R 



especially if it is in excess, or by oxygen (which may be catalyzed by metal 

 ions or enzymes) (Snell and Weissberger, 1939). The degree of oxidation 

 by the unreacted quinone will depend not only on the relative concentra- 

 tions of quinone and thiol, but also on the relative redox potentials of the 

 quinone and thioquinone. In most cases of biological interest, the thio- 

 quinone seems to be the major product, in which case the over-all reaction 

 may be written as: 



HS— R 



Of course, the hydroquinone formed may be reoxidized in various ways, so 

 that the reaction will go entirely to the thioquinone if there is sufficient thiol. 

 A further reaction which often occurs is the addition of thiol to the 

 thioquinone, followed by oxidation as above, to form a dithioquinone, and 

 this process may continue until all available positions are substituted. 

 Thus Blackhall and Thomson (1953) obtained the following products: 



from the reactions of thioglycolate with p-benzoquinone and 1,4-naphtho- 

 quinone. Whether one finds the quinone or the hydroquinone depends on 

 the amount of thiol and its ability to reduce the substituted quinone, as 

 Schubert (1947) demonstrated with the colorless hydroquinone tetrathio- 

 glycolate — if this compound is oxidized to the red quinone form, it is 



