Copyright 2002 Prentice-Hall, Inc. All rights reserved.
Chapter 5 Stereochemistry
Producing a mirror image of a drawing in ChemDraw
A drawing in ChemDraw can be compared to its mirror image by the following procedure:
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Select the drawing (Lasso tool).
Duplicate (Ctrl+drag or Ctrl+Alt+drag to maintain alignment).
With the copy still selected, from the Object menu, choose Flip Horizontal to form the mirror
image in a vertical mirror.
General procedure for searching for chirality
An object is chiral if it is not superimposable on (identical to) its mirror image. Here is a stepby-step description of how this property can be studied in Chem3D:
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Construct a model in Chem3D.
From the Edit menu, choose Select All, and Copy. From the File menu, choose New Model
and Paste (Edit menu) the model into the new window.
From the Tools menu, choose Reflect and Reflect through Y-Z Plane to reflect in a vertical
mirror. Choose Reflect through X-Z Plane to reflect in a horizontal mirror. A mirror image
is a mirror image, but the choice can sometimes affect the ease of model comparison.
Resize the two windows using the window corner and place them side-by-side by grabbing
the title bar of the window and dragging it to where you want it (see Windows Help).
Compare the two models by rotation to see if they can be superimposed (see if they are
identical).
1. Fig 5-4 and Problem 5-2 (d) 2-bromobutane This is a good prototype problem to test the
above procedure.
Chem3D is particularly handy in those cases where a plastic model is difficult to build or where
the molecule contains a large number of atoms. Situations in which Chem3D is probably easier
to use than a molecular model kit are described below.
2. Problem 5-2 (f) Hint: Draw the structure in ChemDraw and import into Chem3D. Select
the Cyclopentane Ring tool and click in the drawing window. Point to one of the carbon atoms
and click again to form the spiro ring system (Wade, Sect 3-16). Select the Solid Bond tool,
double click on the appropriate carbon atoms, and enter capital "O" in the text box to replace the
carbon with oxygen.
3. Problem 5-3 (k) Draw this structure in ChemDraw and import into Chem3D. Use the
Templates tool, click the BICYCLIC.CTP menu, and click the bicyclo[2.2.1]heptane system (in
the upper right-hand corner). Click on the Drawing Area where you want the model to be
deposited. Use the Solid Bond tool to add the double bond and the methyl group.
4. Sect 5-2C Convince yourself that the stereoisomer of 1,4-dibromo-2,5-dichlorocyclohexane
shown is achiral. Use the Chair Cyclohexane tool to place the chair and then add the bromine
atoms and the chlorine atoms at the appropriate position (think up and down at each carbon
atom). Import into Chem3D and follow the general procedure.
Determining R and S
Chem3D can help in the assignment of configuration if it is necessary to reorient the structure
relative to the viewer. Note the Problem-Solving Hint on p. 183. Chem3D provides an
alternative to building a plastic model. The model must be constructed accurately and the
rotation tools used to manipulate the structure so that the lowest priority group is oriented away
from the viewer.
5. The interchanging of any two substituents on a chiral carbon inverts the configuration.
Two interchanges of substituents amounts to the retention of configuration. (ProblemSolving Hint, p. 185) Prove these very useful observations by manipulating a model of 2bromobutane in Chem3D.
6. Problem 5-6 and 5-7 As an alternative to a plastic model, draw the molecule in ChemDraw
and import into Chem3D. Check the stereochemistry! Alternatively the model can be
constructed directly in Chem3D.
7. Problem 5-14 (e) trans-1,3-dibromocyclohexane The planar cyclohexane conformation in
ChemDraw's Templates, CONFORM.CTP menu might be of use in this problem.
8. Problem 5-14 a, b, c These problems can be quickly solved by working with and modifying
one eclipsed conformation of ethane (Templates, CONFORM.CTP).
9. Use the general procedure to verify the chirality of the following species:
a. trans-cyclooctene (Fig. 5-18) Build the model in Chem3D to fix the trans stereochemistry.
b. allenes
Use the Double Bond tool to draw a default structure. Notice that the default structure is planar.
Select one of the double bonds.
Use the Internal Rotation bar along the left side of the model window to rotate about the other
bond. The amount of rotation appears in the Status bar at the top of the window. Rotate 90.0°.
This is the structure of allene, (Fig. 5-19), the simplest system containing cumulated double
bonds. Use the trackball, rotation bars, and spin commands to examine the model from various
perspectives. Note that the bonds at the terminal carbons are in perpendicular planes. (With a CC bond still selected, use the Move to Z Axis to see this clearly.)
Type a capital "C" in the text box from the Text Building tool, select a terminal H atom, and
press enter to replace the hydrogen with a methyl group. Do the same at the other end of the
allene to form 2,3-pentadiene (Sect 5-9B). Examine your structure for chirality.
10. Problem 5-15 (h) Draw in ChemDraw and import into Chem3D.
11. Fischer projections
To draw a Fischer projection of a molecule containing one asymmetric carbon atom, use the
Solid Bond tool of ChemDraw to draw each of the four bonds. Add the substituents at the end of
each bond.
Reproducing the drawings of Fig. 5-20 is a very useful exercise. Consider one of the
enantiomers of 2-chlorobutane and draw a perspective drawing in ChemDraw with two solid
bonds in the plane of the screen. Look down at a pair of bonds directed away from you and
represent the configuration with horizontal bonds directed out of the plane of the screen and the
vertical bonds directed into the plane of the screen. Check your new perspective drawing.
Finally, represent your perspective drawing in the Fischer projection.
Although Chem3D does a pretty good job of recognizing stereochemistry in a ChemDraw
drawing, it is a good idea to check the configuration of any chiral center after importing.
12. Enantiomers and Diastereomers Draw (2R,3R)-2,3-dibromobutane in Fischer projection
(Sect 5-12). Copy and Paste into Chem3D. Examine the model from appropriate perspectives to
verify the configurations. Minimize the energy and note the total. Select All the atoms (Edit
menu) and Reflect through a Y-Z plane (Tools menu). Examine the model from appropriate
perspectives to verify that each of the asymmetric carbons has the S configuration. Minimize
the energy and note the total. Enantiomers have identical physical properties and identical steric
energies.
Select one of the asymmetric carbon atoms. Choose Invert in the Tools menu. Since only one of
the configurations is changed, the structure is not the mirror image of the original and therefore
not an enantiomer and must be a diastereomer. Examine the model from appropriate
perspectives to verify that the structure is (23,3S)-dibromobutane, the meso compound.
Minimize the energy and compare the total to the energy of the enantiomers. Diastereomers
generally have different steric energies and different physical properties.
13. Problem 5-39 Draw the product in ChemDraw and import into Chem3D. Examine the
model for chirality. Note the absence of a chiral center!
14. Hexahelicene is shown below. Draw hexahelicene in ChemDraw. You might have to work
at it! One way is to start with a "deformed" ring such as the one indicated by the arrow in the
structure below.
Use the Benzene Ring tool and click in the drawing window. Select the Solid Bond tool, hold
the Shift key, and drag one of the carbons slightly toward the center of the ring. Do the same to
the opposite carbon. This deformed ring should allow the placement of the other five rings
without "connecting" them. With the Benzene Ring tool selected, click on the bond where you
would like to fuse a new ring (see Wade, Sect 3-16).
Lasso the structure, Copy and Paste it into an empty model window of Chem3D.
Minimize the energy. Examine the structure from various perspectives. Note that it is not
planar! Measure the distance between proximate carbon atoms of the terminal rings.
Finally, use the techniques described at the beginning of this chapter to determine if your
structure is chiral.
15. Problem 5-39 Models of the isomers of 1,2,3-trimethylcyclopentane can be quickly
generated in Chem3D using the methods practiced in this chapter. Use the Invert command to
change the configuration systematically at the three carbons bearing the substituents. Examine
the models for planes of symmetry indicating a meso compound. Reflect the model and
determine if the mirror images are superimposable.