Consequences of Inversion in SN2 Reactions

What are the consequences of the inversion of stereochemistry in the Sn2 reaction Because the reaction is stereospeciflc, we can design ways to use displacement reactions to synthesize a desired stereoisomer. 

Consider the conversion of 2-bromooctane into 2-octanethiol in its reaction with hydrogen sulfide ion, HS . If we were to start with optically pure R bromide, we would obtain only S thiol and none of its R enantiomer. 

Inversion of Configuration of an Optically Pure Compound by Reaction 

Color code for priorities (see Section 5-3) Highest red Second highest blue Third highest green Lowest black 

But what if we wanted to convert (/ )-2-bromooctane into the R thiol One technique uses a sequence of two Sn2 reactions, each resulting in inversion of configuration at the stereocenter. For example, an Sn2 reaction with iodide would first generate (S)-2-iodooctane. We would then use this haloalkane with an inverted configuration as the substrate in a second displacement, now with HS ion, to furnish the R thiol. This double inversion sequence of two Sn2 processes gives us the result we desire, a net retention of configuration. 

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Consequences of Inversion in Sn2 Reactions Further implications of material in Chapters 4 and 5. 

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