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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3hx218

Which of the following give an optically inactive product (ignore partial inversion in SN1; assume complete racemisation where an SN1 step is stated)?

  1. A(S)-1-bromo-2-methylbutane with in DMSO
  2. B(S)-2-bromobutane with in DMSO (SN2)
  3. C(R)-1-bromo-1-phenylethane in aqueous acetone (SN1, benzylic cation)
  4. D(R)-3-bromo-3-methylhexane in water (SN1)

Answer: C, D

The SN1 reactions go through planar carbocations to racemic alcohols with one stereocentre. The SN2 reaction gives pure (R)-butan-2-ol, and the cyanide reaction leaves the stereocentre intact and gives optically active (S)-3-methylpentanenitrile.
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