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