1. Conformations and Stereoisomerism: Rings and Double Bonds
Chapter 4
Conformations and
Stereoisomerism:
Rings and Double Bonds

2. Chapter 4 Problems In-Text Problems 1 - 5 8 - 22 24
End-of-Chapter Problems
25a, b 26a, b a, b a, b , 39, 40, 42, 43, 47, a, c, d, f, h a, c, d
Read Sect. 4.4 and 4.10 (no lecturing)
Read steroid (pp ) and terpenes (pp ) 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 doesn’t 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.

7. 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

9. Ethane: eclipsed conformation
End View
Side View
eclipsed
projection

10. Two conformations of ethane: Newman Projections
ECLIPSED
STAGGERED
rear bonds
stop at edge
bonds on front
carbon meet
only the front
carbon shows H H H H H H H H H H H H
Looking down the carbon-carbon bond.

11. CONFORMATIONS DIFFER IN ENERGY
numerous other
conformations in
between
EXTREMES
12.1 kJ/mol
eclipsed
E N E R G Y
3 Kcal/mol
staggered
0 kJ/mol
0 Kcal/mol
The staggered conformation has a lower energy.

12. 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.

13. Torsional strain versus angle of rotation about the carbon-carbon bond in ethane
etc ……….
eclipsed
3.0 Kcal
3.0 Kcal
Potential
Energy
staggered
ETHANE
Dihedral Angle

14. . PROPANE CH3-CH2-CH3 eclipsed E N E R G Y staggered C1 - C2 CH3 H
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)

15. . BUTANE We will look down the C2 - C3 bond. CH3 C2 - C3 view H
We will look down the C2 - C3 bond.

16. Butane (anti conformation)
end view
side view
no torsional strain or any other strain

17. Butane (gauche conformation)
end view
side view
no torsional strain, but has steric strain

18. 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

19. Butane (syn conformation)
end view
side view
has torsional and steric strain

20. Conformations of butane: Newman projections
CH3 H
ANTI (180o)
CH3 H
SYN (0o)
CH3 H
GAUCHE (60o)
CH3 H
ECLIPSED (240o)
CH3 H
GAUCHE (300o)
CH3 H
ECLIPSED (120o)

21. ( ) Eclipsed always higher energy than staggered! CH3 H CH3 CH3 CH3 H
syn
E C L I P S E D
HIGH ~6
CH3 H
CH3 H
E N E R G Y
eclipsed
3.4
CH3 H
CH3 H
S T A G G E R E D
gauche
1.0
( )
Kcal
mol
CH3 H
anti
LOW

22. Torsional strain (Kj) versus angle of rotation
BUTANE

23. C C X X Y Y Guideline however, 1,2-dihaloalkanes are an exception …..
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 …..

24. 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:
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 won’t work!

25. Sect 4.2: Conformations of halides: molecules with polar bonds
IS ANTI ALWAYS BEST ?

26. 1,2-DICHLOROETHANE Cl-CH2-CH2-Cl WHY….. ?
Prefers the anti conformation in the gaseous state
or in non-polar solvents. Cl H
non-polar
solvent
anti
But, prefers the gauche conformation in polar solvents. Cl H
polar
solvent
gauche
WHY….. ?

27. Cl Cl Cl 1,2-DICHLOROETHANE d- d- d+ d+ d- d+ d+ d- DIPOLE MOMENT bond
dipoles
add to
zero d+ d+ Cl 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 > 0
Preferred conformation in a
non-polar solvent = anti
Preferred conformation in a
polar solvent = gauche