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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3st074

2,3,4-Trichloropentane, , is analysed. Which statements are correct?

  1. AIt has four stereoisomers in total.
  2. BTwo of the stereoisomers are meso and differ in the descriptor (r or s) of the central carbon.
  3. CIn the optically active (2R,4R) isomer, C3 is a stereogenic centre.
  4. DThe rule gives 8, and all eight combinations are distinct compounds.

Answer: A, B

Using the Fischer arrangement of the three C-Cl bonds as left or right gives 8 combinations. Rotating the molecule by pairs them up, so there are 4 distinct stereoisomers. Two (2R,3r,4S and 2R,3s,4S) are meso, because C3 is pseudoasymmetric there, and the other two are the (2R,4R) and (2S,4S) enantiomers. In the (R,R) isomer the two branches at C3 have the same chirality, so C3 is not stereogenic. The 8 combinations are not all distinct.
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