In which of the following reactions does the stereocentre undergo net inversion of its spatial configuration (not merely a change of the letter)?

A(S)-2-bromopropanoate ion with dilute aqueous NaOH (low $[OH X^{-}]$ )
B(S)-butan-2-ol with $PBr X_{3}$
C(R)-octan-2-yl tosylate with $CH X_{3} SNa$ in DMF
D(S)-1-phenylethanol with $SOCl X_{2}$ in a non-nucleophilic solvent with no base (SNi)

Answer: B, C

A secondary tosylate with a good nucleophile in DMF reacts by SN2 and inverts.

PBr X_{3}

makes

- OPBr X_{2}

, which is displaced by bromide in an SN2 step, so it also inverts. The bromopropanoate reacts through an alpha-lactone (two inversions, net retention), and the SNi reaction of the chlorosulfite proceeds with retention.

Practice Organic Reaction Mechanisms III: Nucleophilic Substitution More on SN1 vs SN2: substrate, nucleophile, solvent, leaving group

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In which of the following reactions does the stereocentre undergo net inversion of its spatial configuration (not merely a change of the letter)?

Answer: B, C

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