586 5. QUINONES 



has been a good deal of interest in p-phenylenediamine and its iV-alkyl 

 derivatives, since they arise in the metabolic degradation of carcinogens 

 such as butter yellow (p-dimethylaminoazobenzene) and may be the active 

 agents (Kensler et al., 1942 b). The following discussion does not aim to be 

 complete but only to compare chemically and biochemically these amines 

 and their quinoneimines with the hydroquinones and quinones. 



Chemical Properties 



These amines can undergo oxidation-reduction reactions analogous to 

 the hydroquinone-quinone reaction: 



OH 



— I I + 2 H £o = +0.728 v 



NH2 NH 



/) -Aminophenol /) -Quinoneimine 



+ 2 H £0 = +0-801 V 



NH2 NH 



p -Phenylenediamine p -Quinonediimine 



The redox potentials are somewhat higher than for p-benzoquinone (Fieser, 

 1930 a). Semiquinonediimines are also formed and are often more stable 

 than the semiquinones. Whereas the semiquinones are more stable at al- 

 kaline pH, however, the semiquinonediimine free radicals become more sta- 

 ble as the pH is lowered, since this favors the cationic form (Michaelis, 

 1935). jo- Aminophenol and p-phenylenediamine in solution are oxidized 

 by oxygen, and the rate increases with the pH (Bernheim and Bernheim, 

 1938). Indeed, the quinonoid forms used in metabolic experiments have 

 usually been prepared by allowing solutions of the amines to stand at 

 room temperature or 37^ for 30-60 min. The solutions become dark and this 

 is taken as evidence for the oxidation. However, if this is done around 

 neutrality, one will presumably not end up with much of the quinonoid 

 forms, since they are quite unstable. The half-life of p-quinoneimine at 

 pH 7.57 is 1,1 min and of p-quinonediimine 0.1 min (Fieser, 1930 a). This 



