Stereoisomers

<< Introduction to Organic Chemistry

 

Determine no. of stereoisomers isomers in a compound

 

Prerequisite Knowledge

• Stereoisomerism (geometric/optical)

 

Notes

 

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?

 

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

 

Practice

 

Identify all the chiral C present in the anticoagulant warfarin.