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JEE Main · 2022 · 27 Jun 2022 · Shift 1 · General Organic Chemistry

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Total number of possible stereoisomers of dimethyl cyclopentane is ____

Answer: 6

Solution

Dimethylcyclopentane has three positional isomers:

1,1-dimethylcyclopentane: No chiral centers, no stereoisomers (1 structure).

1,2-dimethylcyclopentane: Two chiral centers. This gives:
  • cis-1,2-dimethylcyclopentane (a pair of enantiomers = 2 stereoisomers)
  • trans-1,2-dimethylcyclopentane (meso compound = 1 stereoisomer)
Total: 3 stereoisomers

1,3-dimethylcyclopentane: Two chiral centers. This gives:
  • cis-1,3-dimethylcyclopentane (meso compound = 1 stereoisomer)
  • trans-1,3-dimethylcyclopentane (a pair of enantiomers = 2 stereoisomers)
Total: 3 stereoisomers

However, the question asks for stereoisomers of "dimethyl cyclopentane" considering only those isomers that show stereoisomerism. 1,2- gives 3 and 1,3- gives 3, totaling 6 stereoisomers.
Structure 1,1-dimethylcyclopentane
1,1-dimethylcyclopentane
Structure 1,2-dimethylcyclopentane
1,2-dimethylcyclopentane
Structure cis-1,2 (enantiomer 1)
cis-1,2 (enantiomer 1)
Structure cis-1,2 (enantiomer 2)
cis-1,2 (enantiomer 2)
Structure trans-1,2 (meso)
trans-1,2 (meso)
Structure 1,3-dimethylcyclopentane
1,3-dimethylcyclopentane
Structure cis-1,3 (meso)
cis-1,3 (meso)
Structure trans-1,3 (enantiomer 1)
trans-1,3 (enantiomer 1)
Structure trans-1,3 (enantiomer 2)
trans-1,3 (enantiomer 2)
Structure Additional stereoisomer
Additional stereoisomer

JEE Main · 2022 · 27 Jun 2022 · Shift 1 · Integer answer

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