Chapter 5: 5.3 R/S Configurations of Chiral Centers

What are two stereoisomers described as?

• Chiral center → stereoisomerism

  • If a molecule has a chiral center (an atom, usually carbon, attached to four different groups), then that atom can exist in two different 3D arrangements.

  • Those arrangements are not interchangeable — you can’t rotate one to make it identical to the other.

• Those two arrangements are stereoisomers

  • “Stereoisomers” = isomers that have the same molecular formula and connectivity but differ in the spatial arrangement of atoms.

• The special case: enantiomers

  • When the two stereoisomers are non-superimposable mirror images, they are specifically called enantiomers.

How do we identify each enantiomer individually?

1. We can use (CIP)

   1. names after chemist Cahn, Ingold, and Prelog

What are the rules when using CIP?

1. You have to assign priority groups

   1. Find the chiral center

   2. Assign the largest atomic number to the as the highest priority (#1)

   3. The smallest atomic number will be assigned the lowest priority (#4)

   4. Now we will assign whether it is

      1. S or R

How do you assign whether it is S?

If it is S it will be counterclockwise

How do you assign whether it is R?

It will be clockwise

How does R and S relate to an enantiomer?

1. If one enantiomer’s chiral center is R, then the other enantiomer’s corresponding corresponding chiral center will be S

2. This is true for every pair of enantiomers

What happens if the lowest priority group (#4) is not on a dash or a wedge?

1. We would have to redraw the molecules to where 4 is a wedge or a dash

   1. If it is S it will turn into R

   2. If it is R it will turn into S

What happens if you are assigning priorities and the chiral centers have two of the same atoms?

You look at the atom next to it to break the tie and whichever has the bigger atomic mass, you assign it to that

Assign the configuration of the chiral center in this compound?

What is a stereodescriptor?

a label that tells us the #D arrangement of atoms at chiral center

What do we have to do when naming a compound with a signle chiral center?

The configuration of the chiral center is indicated at the beginning of the name, italicized, and surrounded by parentheses

What do we have to do when naming a compound with multiple chiral centers present?

You have to put the number followed by a configuration to indicate its location on the parent chain

Ex; In this example each enantiomer has two chiral centers, so two configurations must be provided in the IUPAC name