428 5. QUINONES 



acidic conditions {pK,, = 1.0) and is not physiologically important (Bie- 

 dermann, 1956). The hydroxynaphthoquinones are usually ionized fairly 

 readily; e. g., the p^,^ of lawsone is 3.85, of juglone 8.0 (Friedheim, 1934), 

 and of lapachol 5.02 (Fieser et al., 1948). Thus some of these compounds 

 may exist as anions at physiological pH. The P-ff^'s of various quinones 

 decrease around 0.13 unit with 10^ rise in the temperature (Baxendale and 

 Hardy, 1953). 



Solubility 



The solubilities of the quinones depend on the pH, the salt concentra- 

 tion, and the temperature. p-Benzoquinone and p-benzohydroquinone are 

 soluble in water to the extent of around 120 vaM and 630 vaM, respectively, 

 at 250 (EUinger, 1923; Linderstrom-Lang, 1924; Korman and LaMer, 

 1936). Substitution of p-benzoquinone with methyl groups progressively 

 reduces the aqueous solubility so that duroquinone is poorly soluble. 

 1,4-Naphthoquinone is reasonably soluble in water (22 mM) (Ansbacher et 

 al, 1942) but much less soluble in 0.01 M HCl and 0.09 M KCl (0.67 mM) 

 (Klaning, 1955). Menadione and menadiol are soluble to the extent of 

 7.4 mM and 26 mM, respectively (Ansbacher et al., 1942). Menadione bisul- 

 fite and menadiol diphosphate are very soluble in water, whereas menadiol 

 diacetate is quite insoluble. Many of the biologically important quinones 

 with several hydrophobic groups or long hydrocarbon sidechains are vir- 

 tually insoluble in aqueous media. 



Oxidation-Reduction Reactions 



The P-Q-29-QH2 redox couple has been thoroughly studied because of the 

 use of the quinhydrone electrode for pH measurements and is usually 

 represented by the equation: 



p-Q + 2 H+ + 2e- -? p-QH;, 



This equation describes the oxidation-reduction reaction for all of the 

 quinones except those whose hydroquinones ionize appreciably in the pH 

 range used. The standard oxidation-reduction potential of p-benzoquinone 

 is quite high {E^ = + 0.699 v), which means that it is a potent oxidizing 

 agent, and this is fundamentally due to the restoration of aromatic reso- 

 nance upon reduction. The resonance energy of p-benzohydroquinone is 

 around 36 kcal/mole, but of jj-benzoquinone only 3-4 kcal/mole; the dif- 

 ference in energy corresponds roughly to the energy involved in the ox- 

 idation-reduction reaction. The different redox potentials of the various 

 quinones relate to the resonance energy differences between the quinone 

 and hydroquinone states (Berliner, 1946). The potentials depend, of course, 

 on the pH and in general are 0.40-0.42 v less at pH 7 relative to the 

 standard state (pH = 0) for the benzoquinones; the differences are some- 



