We'll see in Chapter 11 that alkyl halides react with...

We'll see in Chapter 11 that alkyl halides react with hydrosulfide ion (HS $^{-}$ ) to give a product whose stereochemistry is inverted from that of the reactant. Draw the reaction of ( $S$ )-2-bromobutane with HS - ion to yield 2-butanethiol, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{SH}) \mathrm{CH}_{3} .$ Is the stereochemistry of the product $R$ or $S ?$

We'll see in Chapter 11 that alkyl halides react with hydrosulfide ion (HS $^{-}$ ) to give a product whose stereochemistry is inverted from that of the reactant.
Draw the reaction of ( $S$ )-2-bromobutane with HS - ion to yield 2-butanethiol, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{SH}) \mathrm{CH}_{3} .$ Is the stereochemistry of the product $R$ or $S ?$

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Key Concepts

SN2 Reaction Mechanism

The SN2 reaction is a nucleophilic substitution that occurs in one concerted step. In this mechanism, a nucleophile attacks the electrophilic carbon from the side opposite the leaving group, causing the simultaneous bond formation and bond breaking. This attack from the rear leads to a characteristic inversion of stereochemistry at the chiral center.

Stereochemical Inversion

Stereochemical inversion refers to the process by which the arrangement of substituents around a chiral carbon is reversed, much like an umbrella flipping inside out. In the context of an SN2 reaction, the backside attack by the nucleophile results in the inversion of the original configuration, meaning that if the starting material has an S configuration, the product will display an R configuration, all else being equal.

R and S Configuration Assignment

The R and S designations are used to denote the absolute configuration at a chiral center according to the Cahn-Ingold-Prelog priority rules. These rules involve ranking the attached substituents based on atomic number and spatial orientation, and then determining the configuration based on the direction of decreasing priority when the lowest priority group is oriented away from the viewer.

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