Your Name ________________________________
Lab 3 Report Sheets
Partners Name _____________________________
Part A. The Conformations of Acyclic Molecules.
Construct a model of ethane by joining two carbons with a single bond and then attaching three
hydrogens to each carbon.
H
H
H
C
C
or
H
H
H
Expanded formula
H3C
CH3
Condensed formula
Rotate the model around the center bond so that all of the hydrogens are in a staggered conformation.
Various representations of this conformation are shown below.
H
H
H
H
H
H
H
H
H
H
H
H
Newman projection
HH
H
H
"Sawhorse" projection
H
Wedge-Dash bond formula
1. Does the staggered conformation of ethane have any planes of symmetry?
a) yes
b) no
(if answer is “no,” skip to question # 4)
2. How many?
a) 2
b) 3
c) 4
d) 5
H
e) 6
3. Where are they?
a) They are all along the C-C axis, intersecting each hydrogen
b) One is along the C-C axis; one is perpendicular to and bisecting the bond.
c) Two are along the C-C axis; one is perpendicular to and bisecting the bond.
d) Three are along the C-C axis; one is perpendicular to and bisecting the bond.
4. The rotational axes of symmetry are:
a) 3 twofold axes and 1 threefold
b) 6 twofold axes and 1 threefold
c) 3 twofold axes and 2 threefold
d) 4 twofold axes and 2 threefold
5. Is there a center of symmetry for the staggered conformation?
a) yes
b) no
If your answer is yes, where is it? ____________________
Rotate one carbon atom with respect to the other to make the eclipsed conformation.
6. Draw the projection of this conformation using the Newman, “sawhorse” and wedge-dash formulas
in the space below. Label each one.
7. How many planes of symmetry are there in the eclipsed conformation?
a) 2
b) 3
c) 4
d) 5
e) 6
8. Where are they?
a) They are all along the C-C axis, intersecting each hydrogen.
b) One along the C-C axis and one bisecting the C-C axis.
c) Two along the C-C axis and one bisecting the C-C axis.
d) Three along the C-C axis and one bisecting the C-C axis.
e) Three along the C-C axis and two bisecting the C-C axis.
9. The rotational axes of symmetry present are
a) 3 twofold axes and 1 threefold axis
b) 6 twofold axes and 1 threefold axis
c) 3 twofold axes and 2 threefold axis
d) 4 twofold axes and 2 threefold axis
10. Is there a center of symmetry for the eclipsed conformation?
a) yes
b) no
If your answer is yes, where is it? _________________________
11. In which conformation, do the electrons in the C-H bond on one carbon come closest to the electrons
in a C-H bond on the other carbon?
a) staggered
b) eclipsed
12. Considering that like charges repel, which conformation of ethane is less stable?
a) staggered
b) eclipsed
13. In the space below, plot the potential energy of the ethane molecule (y-axis) as a function of the
angle between a C-H bond on one carbon and a C-H bond on the other carbon (this is called the
dihedral angle) as one carbon is rotated about the C-C bond (x-axis). Start with the eclipsed
conformation and go through one full revolution. Label the graph to show where each eclipsed and
staggered conformation is located.
P.E.
0o
60o
120o
180o
240o
300o
360o
Construct a model of propane using three black “balls” and eight white pieces. Arrange all of the
hydrogens in a fully staggered conformation. This means the hydrogens on the first carbon will be
staggered with respect to the hydrogens on the second carbon and those on the second carbon will be
staggered with respect to the hydrogens on the third carbon. See the figure below.
H
H
H
H
H
H
H
H
14. How many planes of symmetry are there in this conformation of propane?
a) 0
b) 1
c) 2
d) 3
e) 4
15. If you found any plane(s) of symmetry, indicate the location(s).
a) There are none.
b) One along the C-C axis and one bisecting the molecule at the central carbon.
c) Two along the C-C axis and one bisecting the molecule at the central carbon.
16. The rotational axes of symmetry present are:
a) There are none
b) 1 twofold axis
c) 2 twofold axes
17. Is there a center of symmetry for the staggered conformation?
a) yes
b) no
If you answer is yes, where is it? _________________________
18. Draw a Newman projection for this conformation, looking down either C-C bond. Let one of the
methyl groups be represented by a “-CH3” in place of an “H” in the staggered conformation of
ethane.
Now rotate the propane model around each of the C-C bonds to make a fully eclipsed conformation.
See the drawing below.
H
H
H
H
H
H
H
H
19. How many planes of symmetry are there in this conformation of propane?
a) 0
b) 1
c) 2
d) 3
e) 4
20. If you found any plane(s) of symmetry, indicate the location(s).
a) There are none
b) One along the C-C axis and one bisecting the molecule at the central carbon.
c) Two along the C-C axis and one bisecting the molecule at the central carbon.
21. The rotational axes of symmetry present are:
a) There are none
b) 1 twofold axis
c) 2 twofold axes
22. Is there a center of symmetry for the eclipsed conformation?
a) yes
b) no
If your answer is yes, where is it? ________________________
23. Draw a Newman projection for this conformation, looking down either C-C bond. Let one of the
methyl groups be represented by a “-CH3” in place of an “H” in the eclipsed conformation of ethane.
24. Just like you did for ethane in exercise 13, plot the potential energy of propane as a function of the
dihedral angle. Do you expect the energy difference between the staggered and eclipsed
conformations to be larger than, equal to, of smaller than the differences between the conformations
of ethane? Indicate this in your plot by making the difference in energy (using the y-axis) larger, the
same size, or smaller compared to your plot for ethane in exercise 13.
P.E.
0
o
60
o
120
o
180
o
240
o
300
o
360
o
25. If you drew the plot in exercise 24 with energy differences greater or less than you did for the ethane
plot in exercise 13, in the space below write a brief explanation why your plots are different.
Construct a model for butane by connecting four carbons in a continuous chain and attaching the
appropriate number of hydrogens to each carbon. Arrange your model in a fully staggered
conformation.
26. Draw a wedge-dash bond formula for this model of butane below.
27. How many planes of symmetry are there in this conformation of butane?
a) 0
b) 1
c) 2
d) 3
e) 4
28. If you found any plane(s) of symmetry, indicate the location(s)
a) There are none.
b) One along the C-C axes and one perpendicular bisecting the central C-C bond.
c) Two along the C-C axes and one perpendicular bisecting the central C-C bond.
d) One along the C-C axes bisecting all of the carbons
29. The rotational axes of symmetry present are:
a) There are none.
b) 1 twofold axis
c) 2 twofold axes
30. Is there a center of symmetry for the fully staggered conformation of butane?
a) yes
b) no
If your answer is yes, where is it? ________________________
31. Draw a Newman projection for this conformation, looking down the C2-C3 bond. Represent the C1
methyl group by a “-CH3” in place of an “H” on the “front” carbon on a Newman projection for
ethane and the C4 methyl group by a “-CH3” in place of an “H” on the “back” carbon of a Newman
projection for ethane. This drawing represents the “anti” conformation for butane.
32. Draw similar Newman projections for the two other staggered conformations for butane. Keep the
methyl group on the “front” carbon in the same position for these two that you used in exercise 31.
33. Notice that the two staggered conformations in exercise 32 appear to be mirror images of each other.
Each is a “gauche” conformation. Are they higher or lower in energy than the “anti” conformation?
a) higher
b) lower
Rotate the model around each of the C-C bonds so that the C-H bonds on each carbon are eclipsed with
respect to the C-H bonds on each adjacent carbon.
34. Draw the wedge-dash bond formula for this model of butane below.
35. How many planes of symmetry are there in this conformation of butane?
a) 0
b) 1
c) 2
d) 3
e) 4
36. If you found any plane(s) of symmetry, indicate the location(s).
a) There are none
b) One along the C-C axes and one perpendicular bisecting the central C-C bond.
c) Two along the C-C axes and one perpendicular bisecting the central C-C bond
d) One along the C-C axes bisecting all of the carbons
37. The rotational axes of symmetry present are:
a) There are none
b) 1 twofold axis
c) 2 twofold axes
38. Is there a center of symmetry for the fully eclipsed conformation of butane?
a) Yes
b) no
If your answer is yes, where is it? ________________________
39. Draw a Newman projection for this conformation, looking down the C2-C3 bond. Represent the C1
methyl group by a “-CH3” in place of an “H” on the “front” carbon of a Newman projection for
ethane and the C4 methyl group by a “-CH3” in place of an “H” on the “back” carbon of a Newman
projection for ethane. This drawing represents the “syn” conformation for butane.
40. Draw similar Newman projections for the two other eclipsed conformations for butane. Keep the
methyl group on the “front” carbon in the same projection for these two that you used in exercise 39.
41. Notice that the two eclipsed conformations in exercise 40 appear to be mirror images of each other.
Are they higher or lower in energy than the “syn” conformation?
a) higher
b) lower
42. Just like you did for ethane in exercise 13 and for propane in exercise 24, plot the potential energy of
butane as a function of the dihedral angle. Label the position in your plot for each of the following
conformations: anti, syn, gauche (2), and the other two eclipsed. Indicate the relative energies in
your plot by making the differences in energy for each conformation (using the y-axis) larger, the
same size, or smaller.
P.E.
0o
60o
120o
180o
240o
300o
360o
Part B. The Conformations of the Cyclohexane Ring.
Construct a model of cyclohexane, a six-membered ring of carbon atoms with two hydrogens attached to
each carbon. Answer the following questions.
1. Is the ring planar or puckered?
a) planar
b) puckered
The ring can be arranged in either the “chair” or the “boat” conformation. See below
chair
(carbon only)
H
H
H
H
H
H
H
H
H
H
boat
(carbons only)
H
H
H
H
H
H
H
chair
(with hydrogens)
H
H
H
H
H
H H
boat
(with hydrogens)
With your model in a “chair” conformation, sight along any C-C bond. Assign the two carbons in the
bond you choose the designations C1 and C2.
2. Are the C-H bonds on C1 and C2 staggered or eclipsed?
a) staggered
b) eclipsed
3. Which of the butane conformations that you drew in Part A, exercises 31-40, most closely
resembles the relationship that you see between the C-H bonds on C1 and C2 (also on C4 and C6) of
the “chair” conformation of cyclohexane when you look down these C-C bonds?
a) Anti
b) syn
c) eclipsed
d) gauche
Note that each carbon atom, one C-H bond lies in the plane generated by four of the carbons and one is
almost perpendicular to the same plane. The positions of each hydrogens in the C-H bonds are labeled
“equatorial” and “axial” respectively.
Equatorial
Axial
H
H
4. How many equatorial hydrogens are there in the “chair” conformation of cyclohexane?
b) 4
c) 6
d) 12
a) 3
5. How many axial hydrogens are there in the “chair” conformation of cyclohexane?
b) 4
c) 6
d) 12
a) 3
6. How many equatorial hydrogens are above the “plane” (more up than down) in the “chair”
conformation of cyclohexane?
b) 4
c) 6
d) none
a) 3
7. How many axial hydrogens are above the “plane” (more up than down) in the “chair” conformation
of cyclohexane?
b) 4
c) 6
d) none
a) 3
8. Draw the “chair” conformation of cyclohexane and clearly label all of the axial hydrogens with an
“a” and all of the equatorial hydrogens with and “e.”
Now CAREFULLY rotate the bonds in the model to make the “boat” conformation. Set it on the table
so that four of the C-H bonds touch the surface. Notice that two of the carbons stick up above the other
four. Label one of these two as C1. As you go around the ring in a counter-clockwise motion, label the
remaining carbons from C2 through C6. Sight along the C2-C3 bond.
9. Is the relationship between the C-H bonds on these carbons (C2 and C3) staggered or eclipsed?
b) eclipsed
a) staggered
10. Which of the butane conformations that you drew in Part A, exercise 31-40, most closely resembles
the relationship that you see between the C-H bonds on C1 and C2 (also on C4 and C6) of the “boat”
conformation of cyclohexane when you look down these C-C bonds?
b) syn
c) eclipsed
d) gauche
a) Anti
11. There are two nonadjacent carbons in the “boat” conformation where the hydrogens attached come
closer to each other than any other two hydrogens on different carbons in this conformation. This is
described as a transannular interaction and since it is found in the “boat” conformation, it is also
called the “bow-sprit” or “flagpole-flagpole” interaction. Which two carbons in your model of the
“boat” have such an interaction?
b) C2 and C5
c) C1 and C4
d) C2 and C6
a) C1 and C3
12. Which conformation, “chair” or “boat” do you think is more stable?
b) the “chair”
a) the “boat”
Conformations of Substituted Cyclohexanes
Arrange your cyclohexane model in the “chair” conformation. Replace one of the equatorial hydrogens
with a marker for a chlorine atom (usually green) on a carbon labeled C1.
13. Draw the structure for this model.
14. Sight along the C1-C2 bond, keeping the “chlorine” pointing up. Are there any “gauche”
relationships with the “chlorine” and any carbons in the ring?
b) yes, with C2
c) yes, with C3
d) yes, with C4
a) no
Now “flip” the conformation through the “boat” and then into the other “chair” conformation.
15. Is the “chlorine” equatorial or axial
b) axial
a) equatorial
16. Again, sight along the C1-C2 bond, keeping the “chlorine” pointing up. Are there any C-C bonds on
C2 which have the “gauche” relationship with the C-Cl bond?
17. Which of the two “chair” conformations of chlorocyclohexane is more stable?
b) the one with the equatorial chlorine
a) the one with the axial chlorine
Place two “chlorine” atoms in equatorial positions on adjacent carbons in the “chair” conformation of
cyclohexane. This model represents 1,2-dichlorocyclohexane.
18. Are the two chlorine in this model cis or trans to each other?
b) trans
a) cis
Now “flip” the ring to the other “chair” conformation.
19. Are the chlorines in axial or equatorial positions?
b) one axial and one equatorial
a) Both axial
c) both equatorial
20. Are the two chlorines on the same side (cis) or on opposite sides (trans) of the carbon ring?
b) opposite side (trans)
a) same side (cis)
21. Which of the two “chair” conformations of the 1,2-dichlorocyclohexanes above do you think is more
stable?
b) the diaxial (second) one
a) The diequatorial (first) one
Now move one of the chlorines so that one chlorine is in an axial position on C1 and one is in an
equatorial position on C2
22. Are the two chlorines in this model cis or trans to each other?
b) trans
a) cis
23. Is this arrangement more or less stable than the conformation in exercise 18 above?
b) less stable
a) more stable
Now construct models for the cis and trans isomers of 1,3-dichlorocyclohexane and 1,4-dichlorocyclohexane. Complete the table below using these two models to guide you.
24. Always start with the chlorine on C1 in the axial position. In each blank in the table, place an “e”
for equatorial or an “a” for axial to indicate the position of the chlorine in question.
Chlorine on C1 is
“a”
1,3-dichloro (cis)
Chlorine on
C3 (or C4)
Change Conformation
Flip to the other “chair”
1,3-dichloro (trans)
Flip to the other “chair”
1,4-dichloro (cis)
Flip to the other “chair”
1,4-dichloro (trans)
Flip to the other “chair”
Chlorine on C1
Chlorine on
C3 (or C4)
Construct a model of cis, cis -1,3,5-trichlorocyclohexane with the chlorines in axial positions. Now
“flip” this model to the other “chair” conformation.
25. What happened to the position of the chlorines?
a) They remain axial
b) They all become equatorial
c) Some remain axial and 1 or 2 become equatorial
26. Write a general statement below describing what happens to the equatorial and axial positions when
you “flip” conformations from one “chair” to the other “chair”