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JEE Advanced levelSingle correct optionDifficulty 3 of 3o3241

The tosylate shown (single enantiomer) is treated with in DMSO to give a nitrile by clean . What is the CIP descriptor of the product, and why?

  1. A(R), because inversion always changes S to R
  2. B(S), because the reaction goes with retention
  3. CRacemic, because the carbon is secondary
  4. D(S), despite spatial inversion, because CH2Cl now outranks CN

Answer: D

The reactant is the (S)-mesylate of 1-chloropropan-2-ol. Inversion gives C[C@@H](C#N)CCl. In the reactant the priorities are OMs > CH2Cl > CH3 > H. In the product CH2Cl (Cl,H,H) outranks CN (N,N,N) because Cl > N at the first point of difference, so CH2Cl > CN > CH3 > H. The incoming group takes the second rank where the leaving group was first, so the geometric inversion is cancelled in the label (RDKit: reactant S, product S). The product is (S)-3-chloro-2-methylpropanenitrile.
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