CHEMICAL PROPERTIES 427 



mann and Franck, 1955), and may possibly be of importance in stabilizing 

 certain structures or altering the reactivity of the quinones. It is interesting 

 to note that lawsone, usually written as 2-h3^droxy-l,4-naphthoquinone, is 

 in tautomeric equilibrium with 4-hydroxy-l,2-naphthoquinone: 



OH 



The 1,4-NQ form is more stable by about 3.8 kcal/mole so that the ratio, 

 (1,4-NQ)/(1,2-NQ), is around 500, and this greater stability may be due in 

 part to the possibility of hydrogen bonding in the 1,4-NQ form. Indeed, 

 Lawsone possesses certain properties indicating that it exists to some ex- 

 tent in the 1,2-NQ form. 2-Amino-l,4-naphthoquinone would presumably 

 also exhibit prototropy. 



Ionization 



The hydroquinones are generally weak acids, as may be seen in the ac- 

 companying tabulation taken from the results obtained by Baxendale 

 and Hardy (1953) at 25^ and ionic strength 0.65. The piiC^ of p-benzosemi- 

 quinone is undeterminable but was assumed to be greater than 11 by 



Michaelis et al. (1938). The ionization of catechol [vK^^ = 9.12 and vK„^ 

 = 12.08 at 30^) is similar to that of hydroquinone. It is evident that the 

 introduction of electronegative groups increases the acidity, as in the 

 phenol series, and this should be taken into account in considering penetra- 

 tion into cells. The approximate concentrations of the ionized species in 

 a 10 m.M solution of jj-benzohydroquinone at pH 7.4 and 37° would be 

 p-QH- = 0.047 mM and p-Q= = 0.0000063 mM. However, with the halo 

 and nitro derivatives, for example, the fractions in the ionized forms will 

 be much greater. The cation, 0=9?=0H+, exists only under strongly 



