Determine no. of Stereoisomers

 

 

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no. of stereoisomers = 2n

 

  • n = no. of stereogenic centers i.e. chiral C + C=C or cyclic rings that can exhibit cis/trans
  • Students make mistakes when they blindly count the total number of C=C
  • Note that C=C within a cyclic ring cannot exhibit cis-trans

 

How to (Quickly) Identify Chiral Carbons

  • Ignore C with multiple bonds - they have less than 4 neighbours
  • Ignore C in benzene rings - benzene C have only 3 neighbours
  • Ignore ‘V-shaped C’ - they are bonded to 2H (same groups)
  • Look for C with ‘peace’ or ‘claw’ shape

  • Check if there are 4 different groups on each of these C

How to check if a C in a cyclic ring has different groups?

 

Example:

 

APPROACH

  1. Draw a line bisecting the C and the two groups in question.

  2. If the structure is symmetrical about the line, the two groups are the same and vice versa.
  • Check groups 1 & 2:

not symmetrical → groups 1 & 3 are different

  • Check groups 1 & 3:

not symmetrical → groups 1 & 3 are different

  • Check groups 2 & 3:

not symmetrical → groups 2 & 3 are different

 

all 4 groups are different → C is chiral