CHEMICAL PROPERTIES 747 



off of one mersalyl and the formation of inorganic mercury from the other, 

 essentially the reverse of the reaction whereby the mercurial diuretics are 

 synthesized (the oxymercuration of alkenes). Such reactions presumably do 

 not occur with the simpler organic mercurials. It has also been shown that 

 diphenethjTiyl mercury and glutathione react to form GS — Hg — SG and 

 phenylacetylene (Tanaka, 1961). Mercurials of the type R2Hg might be 

 expected to be unreactive with SH groups; inasmuch as they are quite 

 toxic, Webb et al. (1950) studied their reactions and found that, although 

 most thiols are not attacked, dithiozne is reacted as follows: 



R— Hg— R + R'— SH -> R— Hg— S— R' + RH 



This type of cleavage of the C — Hg bond occurs at physiological temperature 

 and pH. 



The primary products of the reactions between HgClg and cysteine, gluta- 

 thione, thioglycolate, and other monothiols are the dimercaptides of the 

 type R — S — Hg — S — R, and it is difficult to study the initial formation of 

 a monomercaptide R — S — Hg+ (Shinohara, 1935; Stricks and Kolthoff, 

 1953; Stricks et al., 1954). The reactions between HgClg and dithiols are 

 complex and several types of mercaptide may be formed, as indicated in 

 the tabulation above. The occurrence of cyclic mercaptides and polymer- 

 captide linear complexes will depend mainly on the spatial configuration of 

 the SH groups. The pH apparently plays some role in determining the na- 

 ture of the complexes, since as the pH increases above 2.5, more of the 

 forms Hg2(SG)2 and Hg3(SG)2 appear (Kolthoff et al., 1954). 



We next turn to the problem of the stability of mercaptides and it is im- 

 portant to establish the dissociation constants for the fundamental com- 

 plexes formed in simple reactions, such as the following: 



R— S- + Hg++ ±^ R— S— Hg+ 

 R— S- + R'— Hg+ ±5 R— S— Hg— R' 



Table 7-6 shows a few of the recently determined constants for Hg++ and 

 MM. If one assumes that -pK^ and p^g ^I's similar in magnitude, which is 

 reasonable, it is seen that pJ^j (which applies to the reactions above) lies 

 between 20 and 22, with a mean value of 21.3. This range may thus be 

 taken provisionally as indicating the usual affinity between mercurials and 

 thiols in the absence of competing protons and ligands. Comparison of these 

 values with those in Tables 7-4 and 7-5 shows that the affinity of mercurials 

 for SH groups is far greater than for any other single ligands, and that, in a 

 mixture of thiols and various other complexing ligands, a mercurial will be 

 predominantly associated with the thiols. The variations of pK with the 

 temperature and the ionization of auxiliary groups on the thiol are shown, 

 as summarized from the studies of Stricks and Kolthoff (Table 7-7). Proto- 



