Heterolysis , definition

There is some confusion about the various theoretical and experimental distinctions between the Sjj 1 and SN 2 mechanisms. Since molecularity is the number of molecules necessarily undergoing covalency change during the rate determining step (Ingold, 1969), the unimolecular reaction (SN 1) involves rate determining heterolysis of the R—X bond (kx, Fig. 2) without assistance from nucleophilic attack. This definition is independent of the nature of the first intermediate, which in many cases is probably a contact ion pair rather than a free (i.e. symmetrically solvated) carbocation (see... 

By far the most evidence for electrophilic sulphur is found in the sulphenyl halides (RSCl, RSBr) Although these compounds may in theory react either as sources of RS or X (X = halogen), none of the observed reactions of the sulphenyl halides indicate the latter mode of heterolysis. Kharasch et have presented good evidence for the existence of the 2,4-dinitrobenzene-sulphenium ion (Ar ) in strongly acidic media evidence has also been presented of a strong solvent effect upon the rate of reaction of 2,4-dinitroben-zenesulphenyl chloride and cyclohexene - and of a definite substituent effect in the reaction of this sulphenyl chloride with some substituted styrenes in acetic acid . Such observations are entirely consistent with an electrophilic heterolytic ad tion mechanism involving attack by the sulphenyl chloride in the sense iC -Cl. 

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