CHEMICAL PROPERTIES 433 



hydroqiiinone is slow up to pH 7.8 but becomes very appreciable as the 

 pH is raised above this (LaMer and Kideal, 1924). The rate seems generally 

 to be proportional to (OH-)^. During such reactions and in most mixtures 

 of quinones and hydroquinones there are several species present, as was 

 well shown in the study of LuValle and Weissberger (1947 a, b, c), and in 

 enzyme or tissue preparations it is frequently difficult to determine which 

 are the active forms, since all these species are formed to varying degrees 

 whatever is added initially unless special precautions are taken. 



Stability 



Quinones are generally unstable and in solution often polymerize readily 

 during oxidation in air to give brown amorphous products. o-Benzoqui- 

 none is so highly reactive with water that it is difficult to convert catechol 

 to it in solution, and 1,2-naphthoquinone cannot be recrystallized without 

 undergoing some decomposition, while in acid solutions it dimerizes. The 

 rates of decomposition of p-benzoquinone in various buffers and at different 

 pH's were determined by Meunier and Queroix (1924). The decomposition 

 is usually accelerated above pH 6.5. Half of the quinone goes to the hydro- 

 quinone but the fate of the rest is not surely known. Quinones are more 

 stable in the dark than in the light. Hooker (1936) showed that light induces 

 dehydrogenation at position 3 on 2-hydroxy-l,4-naphthoquinone, this lead- 

 ing to dimerization, while if oxygen is present there is formation of 2,3- 

 dihydroxy-1 ,4-naphthoquinone. 



The antibacterial activity of 2>-benzoquinone in solution falls with time, 

 about 30% of the activity being gone in 2 weeks and 50% in 7 weeks (Cooper 

 and Haines, 1928). The reactions are complex and involve oxidation- 

 reductions and polymerizations. 1,2-Naphthoquinone in dilute alcoholic 

 buffer solutions decomposes completely in 3 hr into equimolar amounts of 

 1,2-naphthohydroquinone and 2-hydroxy-l,4-naphthoquinone (Fieser and 

 Peters, 1931). Such reactions must be borne in mind in storing and using 

 quinones for metabolic studies, and indicate that is it usually advisable to 

 make up solutions immediately before use, and to store these for short 

 times, if necessary, in the dark and at low temperatures under nitrogen. 



Quinone Oj uptake over 30 min (/J) 



P-Q 6 



TQ 4 



P-XQ 1 



1,4-NQ 8 



MD 1 



2,3-Dihydroxy- 1,4-NQ 65 



