Conformations and

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Chapter 4

• Conformations and
• Stereoisomerism
• Rings and Double Bonds

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Chapter 4 Problems
In-Text Problems 1 - 5 8 - 22
24 End-of-Chapter Problems 25a, b 26a, b
27a, b 28a, b 31 - 35 37, 39, 40,
42, 43, 47, 51 52a, c, d, f, h 54a,
c, d Read Sect. 4.4 and 4.10 (no
lecturing) Read steroid (pp 321-324) and terpenes
(pp 349-351) for interest.
3
Homework Assignment

• Molecular Modeling Assignment Spartan
• Download this exercise from my website.
• Note that there are two pieces of material
• Molecular Modeling Instructions
• Molecular Modeling Report Sheet.
• This exercise will be omitted this quarter

4
How to find Spartan on campus computers
1) My Network Places should be on the desktop
on computers in all university computer labs. 2)
Select Novell and look for Mendeleev. If you
find it then work your way through the menus, as
follows mendeleev\data1\programs\PC Spartan
Pro 3) If Mendeleev doesnt appear as one of the
choices, search for it from Novell. 4) Once you
have found Mendeleev, now find PC Spartan
Pro from data1, programs, PC Spartan Pro 5) You
want PC Spartan Pro and not PC Spartan! 6) Click
on WFPro.exe to launch the program
5
Another way to find Spartan Pro on campus
computers

• Click on RUN
• Type in the following
• \\mendeleev\DATA1\PROGRAMS\PCSpartan
  Pro\WFPro.exe
• 4) Click OK
• Click on START

6
Sect 4.1 Conformations

• Different arrangements of atoms within a molecule
  that can be converted into one another merely by
  rotation about single bonds.

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Conformations
Different internal arrangements of the atoms in a
molecule that differ by rotation(s) about one or
more of the single bonds.
CONFORMATIONS ARE NOT ISOMERS
Conformations are different arrangements of the
SAME MOLECULE. The connection pattern of the
atoms does not change.
.
8
Ethane staggered conformation
End View
Side View
Sawhorse projection
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Ethane eclipsed conformation
End View
Side View
eclipsed projection
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Two conformations of ethane Newman Projections
ECLIPSED
STAGGERED
H
H
H
H
H
H
H
H
H
H
H
H
Looking down the carbon-carbon bond.
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CONFORMATIONS DIFFER IN ENERGY
numerous other conformations in between
EXTREMES
12.1 kJ/mol
eclipsed
3 Kcal/mol
ENERGY
staggered
0 kJ/mol
0 Kcal/mol
The staggered conformation has a lower energy.
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Torsional Strain

• Extra potential energy that a molecule possess
  due to the presence of eclipsed bonds.
• Eclipsed ethane has torsional strain because of
  the electron pair repulsions between adjacent
  bonds.
• Staggered ethane has no torsional strain.

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Torsional strain versus angle of rotation about
the carbon-carbon bond in ethane
eclipsed
Potential Energy
ETHANE
staggered
Dihedral Angle
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PROPANE
CH3-CH2-CH3
CH3
C1 - C2
H
eclipsed
ENERGY
3.4 Kcal/mol
(14.2 KJ/mol)
H
CH3
staggered
0 Kcal/mol
As with ethane, the staggered conformation has a
lower energy.
Potential Energy Curve is similar to Ethane
(sinusoidal)
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BUTANE
C2 - C3 view
.
1 2 3
4
We will look down the C2 - C3 bond.
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Butane(anti conformation)
end view
side view
no torsional strain or any other strain
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Butane(gauche conformation)
end view
side view
no torsional strain, but has steric strain
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Steric Strain

• Extra potential energy that a molecule possesses
  because groups are crowded together or are forced
  to approach each other to within the sum of their
  effective radii

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Butane(syn conformation)
end view
side view
has torsional and steric strain
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Conformations of butane Newman projections
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Eclipsed always higher energy than
staggered!
syn
ECLIPSED
6
HIGH
ENERGY
eclipsed
3.4
STAGGERED
gauche
1.0
( )
Kcal
mol
anti
0
LOW
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Torsional strain (Kj) versus angle of
rotation
BUTANE
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Guideline
C
C
X and Y large groups
X
X
Y
When large groups are attached to adjacent carbon
atoms in a chain the anti conformation
is preferred.
Y
Gauche is preferred second.
however, 1,2-dihaloalkanes are an exception ..
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Molecules viewed with Chime

• Click on START, Click on PROGRAMS
• Click on Netscape Communicator (4.7), then launch
  Netscape Navigator
• Using Google, type in the address for the Dept.
  of Chemistry, WWU
• http//www.chem.wwu
• Select, course materials, select WWU virtual
  molecular model set
• You may need the free program, Chime, to run this
  program.
• Note Internet Explorer and Netscape 7.1 wont
  work!

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Sect 4.2 Conformations of halides molecules
with polar bonds
IS ANTI ALWAYS BEST ?
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1,2-DICHLOROETHANE
Cl-CH2-CH2-Cl
Prefers the anti conformation in the gaseous
state or in non-polar solvents.
non-polar solvent
anti
But, prefers the gauche conformation in polar
solvents.
polar solvent
gauche
WHY.. ?
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1,2-DICHLOROETHANE
DIPOLE MOMENT
d-
d-
Cl
bond dipoles add to zero
Cl
d
d
d-
d
NO NET DIPOLE MOMENT
bond dipole vectors add to a positive value
d
d-
PREFERRED CONFORMATION DEPENDS ON SOLVENT
DIPOLE MOMENT 0
DIPOLE MOMENT gt 0
Preferred conformation in a non-polar solvent
anti
Preferred conformation in a polar solvent
gauche