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Molecular Model Lab Exercise - Stereoisomers
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2-Chlorobutane
•
•
Prepare a model of 2-chlorobutane. Place the model on the bench in front of you and, imagining a
mirror standing next to it, construct a model of its mirror image. Pick up the second model and
assure yourself that it cannot be superimposed on the first model. Identify the chirality center (also
called a stereocenter) in one of the models. This atom will be bonded to four different groups.
Now remove two groups from this stereocenter (for instance, methyl and chlorine) and switch their
positions. Note that the two models can now be superimposed—they are identical. Now switch any
other two groups (for instance, methyl and hydrogen) and note you have recreated the original pair
of mirror images (enantiomers).
Question 1: Pick up one model and look at the chirality center from the side opposite the lowest
priority substituent. Draw this 3D bond-line (perspective) view of this molecule using the template on
the left by placing H, Cl, CH3, and CH2CH3 groups on the ends of the bonds. Do the same for the other
isomer, using the template on the right. Label each drawing as R or S.
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The following section is designed to give you practice in going back and forth between your model and
3-dimensional projections drawn on paper. Arrange one of your models (only one of them will work!)
so that it is in the conformation shown below (Structure A). Recall that Newman projections show the
molecule from the front view. Note that the mirror image of this model cannot be arranged to match
this conformation.
CH3
Structure B
H3C
H
H
Structure C
H
Cl
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Structure A
Question 2: Complete two 3D bond-line drawings below by looking at your model from each side of
the C2-C3 bond. The open arrows in the diagram above indicate the side from which you are looking at
Structure A. Structure B is a 3D bond-line drawing looking directly from the left side of Newman
projection A so the C2-C3 bond is in the plane of the paper, with C2 to the right of C3. Structure C is a
3D bond-line drawing looking directly from the right side of Newman projection A, with C2 to the left of
C3. These drawings, labeled B and C, represent other views of structure A. Study the threedimensional relationships between A, B, and C. Note that these are NOT different conformations! They
are different views of the same conformation of one isomer!
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Stereochemistry Model Exercise from Dr. Anderson - 9/11/2016
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Choose the model (enantiomer) of 2-chlorobutane that can be twisted into a conformation that exactly
matches structure D below. It is vital to chose the correct enantiomer. Then, rotate bonds in your model
so as to correspond to Fischer projections E, F, and then G, in turn, as described below. Each Fischer
projection will have two “crosses” with C-2 above and C-3 below. Then you will draw each Fischer
projection. The topmost group of the Fischer projection will be at the top and facing away from you.
The ends of the molecule will be eclipsed in conformations E, F, and G.
C2
C
H
Cl
=
CH3
C
Structure D
C3
H
H
! H 3C
Meaning of a Fischer Projection
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Structure E has the chlorine in the top-most position of the Fischer and the C-4 methyl on the bottom.
Structure F has the C-1 methyl on top, and one of the C-3 H’s on the bottom.
Structure G has the C-2 hydrogen on top and the C-4 methyl group on the bottom.
Note that these Fischer projections are conformational isomers of Structure D and can easily
interconvert. The same is NOT true of your other mirror image model–it could NEVER look
identical to any of the structures D-G.
Question 3: a. Draw E, F, and G as Fischer projections in the space below. b. You can’t draw
conformation D as a Fisher projection. Why not?__________ c. What is the configuration (R or S) of
this molecule? _____
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For the next question, make two models of (R)-2-chlorobutane. On one model, replace the hydrogen
labeled HA by a second chlorine atom to produce Model I; on the other model replace the hydrogen
labeled HB to produce Model II.
H3C
Ha
CH3
H
replace Ha with Cl
Cl
CH3
Cl
H
Hb
R-2-chlorobutane
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H3 C
replace Hb with Cl
H3 C
Ha
CH3
Cl
Hb
Cl
Model I
Model II
Cl
H
Question 4:
a. Are models I and II identical?_____
b. Are they enantiomers?_____
c. Are they diastereomers?_____
d. Name the molecules corresponding to Models I and II by the R,S convention, putting the names to
the right of their Newman projections above. Note: When it is ambiguous which carbon should be #1,
according to the IUPAC rules you should number from the end that gives the R–carbon a lower number
than the S-carbon.
Stereochemistry Model Exercise from Dr. Anderson - 9/11/2016
e. Designate Model I and Model II as erythro (similar groups on the same side of the Fischer projection
when the methyls are at the top and bottom) or threo (similar groups on the opposite side of the
Fischer projection) by writing those designations to the right of their structures above.
Keep Models I and II assembled for now! Make two additional models, of (S)-2-chlorobutane. On one
model, replace the hydrogen labeled HA by a second chlorine atom to produce model III; on the other
model replace the hydrogen labeled HB to produce model IV as shown below.
H3C
CH3
Ha
Cl
replace Ha with Cl
H
H3C
Cl
Cl
Hb
S-2-chlorobutane
CH3
replace Hb with Cl
H3C
Ha
!
CH3
H
Hb
H
Model III
Model IV
Cl
Cl
Question 5
a. Are models III and IV identical?
b. Are they enantiomers?
c. Are they diastereomers?
d. Name Models III and IV, including their R, S convention, again placing the names next to their
Newman projections.
e. Give Model III and Model IV erythro or threo designations.
Question 6: You now have four models (I, II, III, and IV). Label each of them with a piece of tape for
identification. Compare them in pairs (six comparisons total). The possible descriptions are
enantiomers, diastereomers, or identical. Make lists of each of the following, listing the compounds by
model number.
a. all pairs of enantiomers _______________
b. all pairs of diasteriomers __________________
c. all identical compounds __________
Question 7: As determined in question 6c, two of your models will be identical because they represent
the meso structure. Arrange one of the meso models into a conformation in which you can easily see
the plane of symmetry. Note that often not all conformations have a plane of symmetry, but as long as
one conformation does the molecule is meso. Draw the 3D bond-line drawing of this conformation
below.
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Disubstituted Cyclohexanes
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•
The next questions ask whether different dichlorocyclohexanes are chiral structures and/or optically
active. One way to determine whether or not a molecule is chiral is to make a mirror image model
of the structure you wish to evaluate. Then, determine whether or not the original and the mirror
image are identical (superimposable). If they are identical, then the structure is not chiral (not chiral
= achiral). If they are not identical, the molecule is chiral and the stereoisomers are called
enantiomers.
An alternate test (that quickly determines achiral structures) is to look for symmetry elements, such
as a plane of symmetry. A structure with a plane of symmetry is achiral. In order for a compound to
be optically active, the compound must be chiral and one enantiomer must be present in excess of
the other.
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Stereochemistry Model Exercise from Dr. Anderson - 9/11/2016
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Assemble a model of 1,4-dichlorocyclohexane with both Cl atoms in the equatorial position. Then, make
the mirror image of Model A.
Question 8:
a. Draw chair and 3D bond-line structures of A and a 3D bond-line structure of its mirror image.
b. Is model A cis or trans?_______
c. Put stars (*) next to any chirality centers in the 3D bond-line drawing of model A that you drew.
d. If A has a plane of symmetry draw a dotted line through it on the 3D bond-line structure you drew.
e. Is model A chiral? ____
f. Is it meso?_____
g. Would a sample of pure A be optically active?_____
h. Are model A and its mirror image identical or different (enantiomers)? ____________
i. If one Cl were changed to Br, would any of the answers above change? Which ones?
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Assemble another model of 1,4-dichlorocyclohexane with the Cl in the equatorial position on C-1 and a
Cl in the axial position on C-4 (Model B). Also make a model of the mirror image of Model B.
Question 9:
a. Draw chair and 3D bond-line structures of B and a 3D bond-line structure of its mirror image.
b. Is model B cis or trans?_______
c. Put stars (*) next to any chirality centers in the 3D bond-line drawing of model B that you drew.
d. If B has a plane of symmetry draw a dotted line through it on the 3D bond-line structure you drew.
e. Is model B chiral? ____
f. Is it meso?_____
g. Would a sample of pure B be optically active?_____
h. Are model B and its mirror image identical or different (enantiomers)? ____________
i. If one Cl were changed to Br, would any of the answers above change? Which ones?
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Make a model of 1,3-dichlorocyclohexane in which both chlorines are equatorial (Model C). Also make
the mirror image of Model C.
Question 10:
a. Draw chair and 3D bond-line structures of C and a 3D bond-line structure of its mirror image.
Stereochemistry Model Exercise from Dr. Anderson - 9/11/2016
b. Is model C cis or trans?_______
c. Put stars (*) next to any chirality centers in the 3D bond-line drawing of model C that you drew.
d. If C has a plane of symmetry draw a dotted line through it on the 3D bond-line structure you drew.
e. Is model C chiral? ____
f. Is it meso?_____
g. Would a sample of pure C be optically active?_____
h. Are model C and its mirror image identical or different (enantiomers)? ____________
i. If one Cl were changed to Br, would any of the answers above change? Which ones?
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Make a model of 1,3-dichlorocyclohexane in which one chlorine is equatorial and one is axial (Model D).
Make the mirror image of Model D.
Question 11:
a. Draw chair and 3D bond-line structures of D and a 3D bond-line structure of its mirror image.
b. Is model D cis or trans?_______
c. Put stars (*) next to any chirality centers in the 3D bond-line drawing of model D that you drew.
d. If D has a plane of symmetry draw a dotted line through it on the 3D bond-line structure you drew.
e. Is model D chiral? ____
f. Is it meso?_____
g. Would a sample of pure D be optically active?_____
h. Are model D and its mirror image identical or different (enantiomers)? ____________
i. If one Cl were changed to Br, would any of the answers above change? Which ones?
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Make a model of 1,2-dichlorocyclohexane (Model E) in which both chlorines are equatorial. Make the
mirror image of Model E.
Question 12:
a. Draw chair and 3D bond-line structures of E and a 3D bond-line structure of its mirror image.
b. Is model E cis or trans?_______
c. Put stars (*) next to any chirality centers in the 3D bond-line drawing of model E that you drew.
d. If E has a plane of symmetry draw a dotted line through it on the 3D bond-line structure you drew.
Stereochemistry Model Exercise from Dr. Anderson - 9/11/2016
e. Is model E chiral? ____
f. Is it meso?_____
g. Would a sample of pure E be optically active?_____
h. Are model E and its mirror image identical or different (enantiomers)? ____________
i. Flip model E to the other chair conformation. Is model E’s other chair form identical to the mirror
image of model E?_____
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Make a model of 1,2-dichlorocyclohexane (Model F) in which one chlorine is equatorial and the other is
axial. Make the mirror image of Model F
Question 13:
a. Draw chair and 3D bond-line structures of F and and a 3D bond-line structure of its mirror image.
b. Is model F cis or trans?_______
c. Put stars (*) next to any chirality centers in the 3D bond-line drawing of model F that you drew.
d. If F has a plane of symmetry draw a dotted line through it on the 3D bond-line structure you drew.
e. Is model F meso? ______
f. Is model F chiral? _____
g. Would a sample of pure F be optically active?_____
h. Are model F and its mirror image identical or different (enantiomers)? ____________
i. Flip model E to the other chair conformation. Is model E’s other chair form identical to the mirror
image of model E?_____
Stereochemistry Model Exercise from Dr. Anderson - 9/11/2016