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Stereochemistry

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1. Stereochemistry. Chapter 4. 2. Stereoisomers. Cis/trans-isomers. E/Z-isomers: these are stereoisomers, but a better term is geometrical isomers. 3 ... – PowerPoint PPT presentation

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Title: 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

8
Enantiomers what do they do?
9
The R/S convention
  • The Cahn-Ingold-Prelog rules revisited determine
    the priority of the four substituents

10
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

13
Other examples
  • Make problem 4.11

14
Two stereogenic centers
15
2 stereocenters, 4 stereoisomers
16
Enantiomers vs diastereomers
17
From 2 to 3 stereocenters
  • With 3 stereocenters, there are 23 8 different
    stereoisomers
  • These 8 stereoisomers consist of 4 pairs of
    enantiomers

18
Meso-compounds
19
meso-compounds are symmetrical
20
Problems
  • Make problem 4.17 and 4.19

21
Stereochemistry in rings
  • Chlorocyclopropane is symmetrical and therefore
    achiral

22
Symmetrical or not?
  • Which of these two are chiral?

23
Explanation
24
Problems
  • Make problems 4.37, 4.44, 4.45, 4.46 and 4.49
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