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Stereochemistry.

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Title: Stereochemistry.


1
Chapter 4
  • Stereochemistry.
  • Stereochemistry of open chain alkane.
  • Stereochemistry of ring alkane.

2
Stereochemistry
Spatial arrangement of atoms in a molecule or ion.
Function and reactivity of a molecule is
determined by its 3-dimensional geometry.
3
Chapter 4
  • Stereochemistry.
  • Stereochemistry of open chain alkane.
  • Stereochemistry of ring alkane.

4
Stereochemistry of open chain alkane.
  • Conformational Isomers vs. Constitutional
    Isomers.
  • Modes of display - Sawhorse, Wedges Dashes
    representations, and Newman projection.
  • Staggered, eclipsed, torsional or dihedral angle,
    torsional strain, anti and gauche conformation,
    steric strain.

5
Conformational Isomers vs. Constitutional Isomers
Constitutional isomers
Conformers
6
Modes of display
7
Ethane
? 0o
Dihedral angle ? 60o
Eclipsed
Staggered
At room temperature there is sufficient thermal
energy for molecules to rotate among the
different conformations.
8
Ethane
2.4Å
2.4Å
Staggered
9
Ethane
2.2Å
Eclipsed
10
Ethane
11
Ethane
3(H-H) 12 kJ/mol
12 kJ/mol
E
Each(H-H) (12 kJ/mol)/3 4 kJ/mol
12
Propane
Staggered
13
Propane
14
Propane
H
H
H
2(H-H) 2x4 8 kJ/mol
CH3
H
H
14 kJ/mol
Eclipse (CH3-H) 14-8 6 kJ/mol
E
(CH3-H) 3.8kJ/mol
15
Butane
16
Butane
17
Higher straight Chain Alkanes
Zigzag with some gauche conformations.
? ? ?
? ? ?
18
Problems
  • Draw Newman projections of the most stable and
    least stable conformations of 3-Methylhexane.
  • Using wedges and dashes to represent the bonds of
    the most stable conformation of 3-Methylhexane.
  • Construct a qualitative potential energy diagram
    for rotation about the C-C bond of
    1,2-Dibromoethane. Include the Newman
    projections of the conformations at the maximum
    and minimum points. Also give the names of the
    various conformations.

19
Chapter 4
  • Stereochemistry.
  • Stereochemistry of open chain alkane.
  • Stereochemistry of ring alkane.

20
Strains
  • Torsional - due to eclipsing of bonds on
    neighboring atoms.
  • Steric - due to repulsive interactions when atoms
    approach each other too closely.
  • Angle - due to expansion or compression of normal
    hybridized bond angles - sp, 180o sp2, 120o
    sp3, 109o.

21
Strains in Alkanes
  • Straight Chain
  • Torsional
  • Steric
  • Ring
  • Torsional
  • Steric
  • Angle

22
Cyclopropane
angle
torsional
23
Cyclobutane
Non-planar
More torsional strain due to more atoms.
Less angle strain
24
Cyclopentane
Non-planar
More torsional strain.
No angle strain
25
Cyclohexane
  • Drawing.
  • Ring flip.
  • Boat and chair form.
  • Axial vs. equitorial.
  • Use of Alchemy.

26
Monosubstituted Cyclohexane
  • 1,3 diaxial interaction.
  • Axial equitorial equilibrium calculation.
  • Use of Alchemy

27
Disubstituted Cyclohexane
  • 1,2-Disubstituted cyclohexane.
  • 1,4- Disubstituted cyclohexane.
  • Use of Alchemy

28
Policyclic rings
  • Fused rings stereochemistry.
  • Use of Alchemy
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