Unit 10

Stereocenter

An atom within a molecule where switching any two of its substituents gives a new stereoisomer

Assigning Absolute Configuration of an Alkene

1. Split the alkene down the middle (perpendicular to the sigma bond)

2. Rank the priority of R1 vs R2 using the CIP rules

3. Rank the priority of R3 vs R4 using the CIP rules

4. Assign E or Z

Z

Together

E

Opposite

CIP Rules

1. Look at the atoms directly attached to each of the double bond carbons and rank them in order of decreasing atomic number

2. If a decision can’t be reached by ranking the first atoms in the substituent, look at the second, third, fourth, etc. atoms away from the double bond carbons until a difference is found.

3. Multiple bonded atoms are considered equivalent to the same number of single bonded atoms

Assign E, Z, or neither

Z

What is the IUPAC name of the alkene above?

(Z)-1-bromo-1-chloro-2-fluoro-2-iodoethene

Asymmetric Centre

All asymmetric centers are stereocenters, but not all stereocenters are asymmeteric centres

Absolute Configurations of Asymmetric Centres (R and S)

1. Prioritize substituents using CIP rules

2. Orient molecule so that #4 priority is pointing straight back into the page

3. Draw an arrow from priority #1 to #2 to #3

R Configuration

Arrow goes clockwise

S Configuration

Arrow goes counter-clockwise

Label each asymmetric centre in the molecule below as R or S.

R