Stereochemistry

1
Stereochemistry

• Chapter 4

2
Stereoisomers

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

3
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

4
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

5
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)

6
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

7
Drawing enantiomers

• Make problem 4.1

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Enantiomers what do they do?
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The R/S convention

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

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Problem 4.2

• Identify the priorities of compounds a-d

11
Dealing with multiple bonds

• Reminder multiple bonds are treated as
  multiplied single bonds

12
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

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Other examples

• Make problem 4.11

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Two stereogenic centers
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2 stereocenters, 4 stereoisomers
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Enantiomers vs diastereomers
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From 2 to 3 stereocenters

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

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Meso-compounds
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meso-compounds are symmetrical
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Problems

• Make problem 4.17 and 4.19

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Stereochemistry in rings

• Chlorocyclopropane is symmetrical and therefore
  achiral

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Symmetrical or not?

• Which of these two are chiral?

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Explanation
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Problems

• Make problems 4.37, 4.44, 4.45, 4.46 and 4.49