Stereoisomers

1
Stereoisomers

• Cis/trans-isomers
• E/Z-isomers these are stereoisomers, but a
  better term is geometrical isomers

2
Mirror images are enantiomers

• Carbon atoms bearing four different substituents
  are called asymmetric centers or stereocenters or
  stereogenic centers
• A compound that contains a stereocenter is called
  chiral

3
Defenition of enantiomers

• Chiral compounds with one stereocenter exist as
  two different stereoisomers which are enantiomers
  i.e. they are mirror images of each other
• Another definition of a chiral compound is a
  compound that does not contain a plane of symmetry

4
In other words..

• 3-methylhexane four different substituents,
  therefore chiral (note that there is no plane of
  symmetry)
• 3-methylpentane two substituents are equal,
  therefore achiral (note that the molecule
  contains a plane of symmetry)

5
The polarimeter

• An enantiomerically pure compound is able to
  rotate the plane of plane-polarized light,
  leading to levorotary (-) or dextrorotary ()
  values
• This does not happen with a racemic (11) mixture
  of the two enantiomers

6
Drawing enantiomers
7
Enantiomers what do they do?
8
The R/S convention

• The Cahn-Ingold-Prelog rules revisited determine
  the priority of the four substituents

9
Problem 4.2

• Identify the priorities of compounds a-d

10
Dealing with multiple bonds

• Reminder multiple bonds are treated as
  multiplied single bonds

11
R or S ?

• Put the substituent with the lowest priority to
  the back
• Going down in priority runs counterclockwise
  S-isomer
• Going down in priority runs counterclockwise
  R-isomer

12
Other examples
13
Two stereogenic centers
14
2 stereocenters, 4 stereoisomers
15
Enantiomers vs diastereomers
16
From 2 to 3 stereocenters

• With 3 stereocenters, there are 23 8 different
  stereoisomers
• These 8 stereoisomers consist of 4 pairs of
  enantiomers

17
Meso-compounds
18
meso-compounds are symmetrical
19
Stereochemistry in rings

• Chlorocyclopropane is symmetrical and therefore
  achiral

20
Symmetrical or not?

• Which of these two are chiral?

21
Explanation