What is the stereochemical outcome when optically pure (2R)-2-bromobutane undergoes an nucleophilic substitution reaction with aqueous hydroxide ion ()?
Answer and explanation
Correct answer: C. Complete inversion of configuration (Walden inversion) yielding (2S)-butan-2-ol
- Step 1: The mechanism proceeds via a concerted, one-step bimolecular pathway.
- Step 2: The incoming hydroxide nucleophile attacks the electrophilic carbon from the backside directly opposite to the leaving bromide group ( attack) through a pentacoordinate transition state.
- Step 3: This backside attack causes an umbrella-like inversion of the remaining three groups (Walden inversion), converting the (2R) configuration cleanly into (2S)-butan-2-ol.
Answer
Complete inversion of configuration (Walden inversion) yielding (2S)-butan-2-ol.
Why not the others: Complete retention requires a two-step process with double inversion or neighbouring group participation; Complete racemization is characteristic of an mechanism involving a planar carbocation intermediate; Formation of a meso-diol would require two chiral centers and multiple substitutions.
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