This secondary reaction may occur via a transient dithiane disulfonium ion intermediate (5-7):
It should be emphasized that all the sulfonium salts (5-6), especially the 2-chloroethyl compound, possess noteworthy toxicity. This toxicity may be due to the decomposition of the sulfonium salts under physiological conditions to form alkylating moieties. The conversion of these sulfonium salts to reactive species is considerably slower than for sulfur mustard. The chemical reactions of the sulfonium salts have been studied in detail, but it is not known whether they are actually formed in vivo . It is certainly possible that such toxic products might be formed on moist areas of the skin, which is consistent with the high susceptibility of these regions to the vesicant action of sulfur mustard. The physiological effects and toxicities of the sulfonium salts need to be investigated, since the proposed mechanism of the cytotoxicity of sulfur mustard is based on the simplified SN1 hydrolysis and is not fully understood.
The relative affinities of nucleophiles are quantitatively described by their competition factors, which compare the rate of constants for bimolecular reactions of cyclic ethylene sulfonium ion with a given nucleophile (Ka) and water (Ko), respectively (5-8):
The dimensions of Fa are 1/concentration, so the reciprocal of Fa is the concentration of nucleophile that must be present in water so that it reacts with 50 percent of the sulfur mustard. An extensive list of