I was going over synthesizing specific enantiomers using SN2 reactions and my text talks about using a 2 step method for generating a R enantiomer (Assume I want to create (R)-2-Octanethiol from optically pure (R)-2-Bromooctane). The text follows a backside displacement of the Br by I in the first SN2 reaction and then the product ((S)-2-Iodooctane) reacts with HS- (another backside displacement) to form (R)-2-Octanethiol.
My question is cant we just follow a one step frontside displacement where HS- displaces Br- since frontside displacement reactions leave the product with the same configuration. If this is not possible, why is it not possible?
Any help is appreciated!!