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JEE Main levelSingle correct optionDifficulty 2 of 3o3143

(R)-1-Chloropropan-2-ol, , is treated with one equivalent of NaOH to give 2-methyloxirane (propylene oxide) by an intramolecular SN2 ring closure. The product is

  1. A(R)-2-methyloxirane, because the stereocentre is not attacked and keeps its geometry
  2. B(S)-2-methyloxirane, because ring closure inverts the stereocentre
  3. Cracemic 2-methyloxirane, because the alkoxide attacks a secondary carbon
  4. D(S)-2-methyloxirane, because the Cl-bearing carbon is the stereocentre and is inverted

Answer: A

The alkoxide oxygen attacks the primary carbon bearing Cl, which is not a stereocentre, so the carbinol carbon keeps all its bonds and its geometry. Priorities are in both the chlorohydrin (CH2Cl) and the epoxide (CH2-O), so the descriptor is also unchanged: R gives R.
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