Which sequence converts (R)-butan-2-ol into (S)-butan-2-ol, with a single net inversion at the stereocentre? (Assume every substitution step is a clean SN2 reaction.)

A $PBr X_{3}$ ; then $CH X_{3} COONa$ in DMF; then aqueous NaOH to hydrolyse the ester (acyl-oxygen cleavage)
BTsCl, pyridine; then LiBr in DMF; then aqueous NaOH acting as an SN2 nucleophile on the bromide
CTsCl, pyridine; then $CH X_{3} COONa$ in DMF; then aqueous NaOH to hydrolyse the ester (acyl-oxygen cleavage)
D $Ac X_{2} O$ , pyridine; then aqueous NaOH

Answer: C

Tosylation and ester hydrolysis break only the O-H or acyl-O bonds and leave the carbon configuration unchanged; only SN2 steps at the stereocentre invert it. The correct route has one SN2 step (acetate on the tosylate), so inversion occurs once. The

PBr X_{3}

route and the LiBr/NaOH route each contain two SN2 steps (net retention), and the acetylation/hydrolysis route has none.

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

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Which sequence converts (R)-butan-2-ol into (S)-butan-2-ol, with a single net inversion at the stereocentre? (Assume every substitution step is a clean SN2 reaction.)

Answer: C

More questions on SN1 vs SN2: substrate, nucleophile, solvent, leaving group