Substitution via Two Sn2 Reactions

When halogen atoms are attached to a vinylic carbon and also to one allylic to it, an SN2 process converts the vinylic halogen into an allylic one, while the formerly allylic one is replaced, and a new olefin is formed. Another SK2 attack at the new terminal vinylic carbon would result in the replacement of the original vinylic halogen. The vinylic halide can thus be exchanged in two consecutive SN2 reactions. 

This mechanism was suggested for the reaction of 1,3-dichlorotetra-fluoropropene with F which gives 1,1,1,2,3,3,3-heptafluoropropane, by nucleophilic addition of F to the substitution product equations 25-27) (Miller et al., 1960). 

The reversible fluoride ion-catalysed rearrangements of perfluoro-olefins (Miller et al., 1960) may also cause vinylic fluorine—fluorine exchange by a similar mechanism. 

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