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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3hx215

Select the SN2 reactions in which the product has a CIP label opposite to that of the substrate. In each case the nucleophile replaces the halogen with inversion at the carbon bearing the halogen; note that this carbon is not always the stereocentre.

  1. A(S)-1-bromo-2-methylbutane with in DMSO (substitution at the carbon)
  2. B(R)-2-bromobutane with in dry acetone
  3. C(S)-2-chlorobutane with sodium ethanoate in DMF
  4. D(R)-2-bromo-1-methoxypropane with in DMSO

Answer: B, C

First two: nucleophile and leaving group have the same priority rank, so inversion flips R to S or S to R. In the methoxy case, the CN group ranks below , so the label stays R. In the last case the stereocentre is not touched, so the label stays S.
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