CHEMISTRY 212
SPRING 2013
Conformational Analysis-1
A. References: CGW Ch. 16: pp. 360-366
B. Acyclic Compounds:
1. Conformations: Structures that differ in the positions of atoms due
to rotation of -bonds.
2. Method of Representation - Newman Projections:
CH3
4 CH
Cl
3
Looking down
H
Cl
H
the bond from
2
3
H
H
H
C2 <- C3
1CH3
H
CH3
R-2-chlorobutane
Cl
H
CH3
The back carbon
atom (C2) and its
three other bonds
CH3
H
H
The front carbon
atom (C3) and its
three other bonds
3. Ethane
CH3 "Free Rotation" around the C-C bond.
\ There is an infinite # of conformations
HH
H
H
H
Dihedral
angle of 60˚
HH
H
H Dihedral
H
H
angle of 0˚
H
Dihedral Angle:
Staggered
Eclipsed
The angle between
Easily recognizable
the substituents
Extreme
Conformations
on adjacent
carbon atoms.
CH3
4. Conformational Analysis of Acyclic Compounds:
a. In Model 1: Complete the Newman Projections of ethane with dihedral angles of 120o, 180o, 240o, 300o & 360o. (See p. 2)
b. What changes occur in the energy of ethane (Model 1) as a C-C bond rotates?
c. What changes occur in the molecular structure of ethane (Model 1) as a C-C bond rotates? Models would be helpful here.
d. Suggest an explanation for the change in energy as bonds rotate.
Conformational Analysis-1
Hxa
H
Model 1
Model 2
Ethane
Energy Diagram for Rotation
Around the C-C bond of Ethane
Butane
Energy Diagram for Rotation Around the C2-C3 Bond of Butane
H
HH
HH
2
Ha
H
Hx
H HH
Ha
H
H
HxH
H
Ha
Hx
H
H HHx
HHa
Hx
HH
CH33
CH
Ha
H
H Hxa
HHH
H
H
H
H H
HH
CH3
CH3
CH3
CH3
CH3
HCH3
CH3
H
H
H HH
H H
H H
H
H H
H 3H
CH
H
CH3
4.5 Kcal/mole
5.3 Kcal/mole
3.0 Kcal/mole
E
E
3.6 Kcal/mole
2.8 Kcal/mole
0
60
120
180
240
300
Dihedral Angle Between Ha & Hx
360
0
0.8 kcal/mole
60
120
180
240
300
Dihedral Angle Between CH3's
e. In Model 2: Complete the 3 additional Newman Projections of butane with dihedral angles of 240o, 300o & 360o.
f. How does the Energy Diagram of Butane (Model 2) compare with that of Ethane (Model 1)?
g. Suggest an explanation for the differences between the rotation diagrams for ethane and butane and provide your warrant.
360
3
Conformational Analysis-1
Reflector’s Report Discussion:
Identify the most important concepts you learned from this activity:
What questions remain?
Strategy Analyst’s Report Discussion:
How did the use of Newman Projections with ethane and butane help you group formulate a hypothesis on interactions between electron clouds
on groups that are not attached to each other in a molecule.
How did this activity affect your understanding of how “free” rotation is around -bonds?
Out of Class Applications:
A. Read: CGW Ch. 16: pp. 360-366
B. Application Questions:
1. Draw a potential energy diagram like those on p. 2 of this activity for rotation around the C2-C3 bond of 2,3-dimethylbutane. Estimate the
relative heights of the energy barriers from those in Models 1 & 2.
2. How does your diagram for 2,3-dimethylbutane compare with the diagram for butane (Model 2, p. 2 of this activity)?