Thiolates thiosulfates

Sodium tetrathionate (Na2S406) is a redox compound that under the right conditions can facilitate the formation of disulfide bonds from free sulfhydryls. The tetrathionate anion reacts with a sulfhydryl to create a somewhat stable active intermediate, a sulfenylthiosulfate (Fig. 102). Upon attack of the nucleophilic thiolate anion on this activated species, the thiosulfate (S203 =) leaving group is removed and a disulfide linkage forms (Pihl and Lange, 1962). The reduction of tetrathionate to thiosulfate in vivo was a subject of early study (Theis and Freeland, 1940 Chen et al., 1934). 

The thiolates, though less sensitive to basicity, are more reactive than oxygen anions over the total accessible range of basicity, but intersect the amine line at ca. pA 12. Other reactive nucleophiles which do not fall in the amine, thiolate, or oxygen anion categories are fluoride, thiosulfate, nitrite, azide, and sulfite. Halides other than fluoride, and also thiocyanate, nitrate, sulfate, and thiourea have no reactivity towards p-nitrophenyl acetate (Jencks and Carriuolo, 1960a). The total lack of reactivity of thiocyanate, iodide, bromide, and thiourea, all very polarizable nucleophiles which are reactive to sp carbon, rules out any possibility that polarizability is at all important in nucleophilic reactions at the carbonyl carbon. In general, the order of nucleophilic reactivity to p-nitrophenyl acetate correlates well with nucleophilic reactivity to other carboxylic acid derivatives (see later). Nitrite, however, shows... 

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