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JEE Main levelSingle correct optionDifficulty 2 of 3o3133

Optically pure (S)-2-bromopropanoate ion, , is left in water with a low concentration of hydroxide (so that the external nucleophile is poorly available). The 2-hydroxypropanoate (lactate) ion formed is mainly

  1. A(R)-lactate, formed with inversion in a single SN2 step
  2. Bracemic lactate, formed through a free carbocation
  3. C(S)-lactate, formed with retention through an alpha-lactone intermediate
  4. D(S)-lactate, formed by direct front-side attack of water

Answer: C

The carboxylate oxygen displaces bromide from the back (first inversion), forming a three-membered alpha-lactone; hydroxide or water then opens the ring at the same carbon from the back (second inversion). Two inversions give retention. Priorities are for both, so the S descriptor is retained.
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