STEREOISOMERISM WITH MOLECULAR MODELS

You will use molecular models regularly in your course to view three-dimensional structures as an aid to predicting behavior and reactivity. This exercise is designed to familiarize you with stereoisomerism. As you build the models below, questions are posed that you should answer in your notebook. You will need to show your work to your instructor for credit before leaving. Your instructor may provide a key.

Enantiomers and Diastereomers

Review the definitions of chirality center, enantiomers, diastereomers, meso, and the symmetry elements (plane, axis, and center of symmetry). A chiral molecule cannot have a plane of symmetry, a center of symmetry, or an axis of symmetry greater than two-fold (i.e., no less than 180° rotation). Learning to recognize these elements, or the lack of them, is a skill best developed with models. 1. Build a model of a tetrahedral carbon with three different groups attached. (For consistency with these instructions use black, red, and two green atoms for these groups and use bonds of all the same length.) What elements of symmetry does this model have? 2. Is this model chiral? 3. Build a second model that is a mirror image of the first. Compare the models; are these models superimposable? 4. If these models were actual molecules, how would you describe the relationship between them? 5. Replace one green atom on each model with a blue atom such that they are mirror images of each other. What elements of symmetry do the models have? 6. Are the models superimposable? 7. Are the models chiral? 8. How would you describe the relationship between molecules represented by these models? 9. Use the two models you have already built for this exercise. Exchange the positions of the green and blue atoms on one of the models so that the two models are now identical (superimposable). Remove the black substituent from each model and join, with a single bond, the two carbons that still have three substituents on each. How many chirality centers does this model have? 10. What elements of symmetry does this model have? 11. Is the molecule chiral? 12. Construct a mirror image of the last model made and then compare the two models. Are the models superimposable? 13. If these models were actual molecules, how would you describe the relationship between them? 14. Can one enantiomer be chiral and the other achiral? Why or why not? 15. Take one of the two models you have already built. Switch the positions of the green and blue substituents that are attached to the same carbon; leave the other carbon unchanged. Now compare the two models again. Are the models still stereoisomers? 16. Are they mirror images? 17. If these models were actual molecules, how would you describe the relationship between them? 18. Now focus on the one model whose substituents you switched. Does this model have any chirality centers? How many? 19. What elements of symmetry does this model have? 20. Is this model chiral? Explain why or why not. 21. Draw a dash-wedge representation of this model in both an eclipsed and a staggered conformation, as shown. Assume that red = oxygen, green = chlorine, blue = nitrogen, and black = carbon. Identify any chiral centers with an asterisk. Rank the substituents on each chirality center and assign each chirality center its correct absolute configuration (R or S).

Organic Chemistry Lab Manual Staggered Eclipsed Harper College, Fall 2010

22. What is the term used to describe a stereoisomer like the one you drew in the previous question? 23. Do molecules with chirality centers have to be chiral? 24. Under what circumstance is a molecule with chirality centers required to be chiral? 25. What test(s) could you do to decide if a molecules has an enantiomer?

Fischer Projections

A Fischer projection is another way to represent asymmetric carbons with simple crossed lines, if we make some fixed assumptions. The intersection of the crossed lines is assumed to be a carbon atom in the plane of the paper (or whatever drawing surface being used). The horizontal lines are always s assumed to be bonds that come out of the plane, toward the viewer. Vertical lines are assumed to be bonds that go behind the plane, away from the viewer. This is illustrated below by showing the same molecule represented with four different drawings, the last drawing being a Fischer projection.

Rotation in Corresponding Cl Cl space to dash-wedge representation Fischer orientation of Projection Fischer view O N O N

C C

26. Compare Fischer projections A and B shown below. Projection B appears to be a 90° clockwise rotation of projection A. Use what you have learned about Fischer projections to build models of the molecules represented by projections A and B, then compare the two models. What is the relationship between these two molecules?

Cl O C

O N C Cl N O

C N Cl Projection A Projection B Projection C

27. Do projections A and B represent the same molecule? 28. Compare projections A and C. What is the relationship between those two molecules? If you cannot decide from the drawings, build models of each.

Challenging Structures

29. The structure to the right has cumulated double bonds. Molecules with this type H CH3 of carbon-carbon double bond relationship are called allenes. What is the CCC hybridization of the center carbon? H3C H 30. On one end of the molecule, the hydrogen, carbon, and methyl carbon define a plane. The corresponding atoms at the other end of the molecule define a different plane. What is the geometric relationship between these two planes? 31. Build a model of the allene shown. What elements of symmetry does this molecule have? 32. Does this molecule have any chirality centers? Is allene chiral? 33. Build the mirror image of the allene model and compare it to the original. Are they superimposable? 34. What term would be used to describe the relationship between these two molecules? 35. Is a chirality center required in order for a molecule to be chiral? Organic Chemistry Lab Manual Harper College, Fall 2010