In the Classroom
W
The Use of Stick Figures to Visualize Fischer Projections
Laurie S. Starkey Chemistry Department, California State Polytechnic University, Pomona, Pomona, CA 91768-4032;
[email protected] While many techniques have been developed for the interconversion between Fischer and Haworth projections (1–4), few methods for the conversion between line drawings and either Fischer projections (5, 6 ) or Haworth drawings (7) have been described in this Journal. The technique described herein helps students envision 3-D structures by employing simple stick figures (8) to see 2-D drawings from the proper point of view. Imagine your body suspended in the plane of the page and aligned with a line drawing. By viewing the structure from above or below, you would have the proper perspective to easily complete a Fischer projection. This seems obvious to most readers, since this technique comes naturally to us after years of experience; we can just “see” it. The challenge lies in specifically training beginning students to develop these same visualization skills. Drawing a cartoon eye to represent a point of view is common practice in textbooks. However, a single eye looking “down” does not give the complete perspective or point of view. By using an entire stick figure to represent the viewer’s body, we can not only make reference to looking “down” at the structure, but we can also ask, “Which groups are aligned with my head and feet?” and “Which groups are on my left and right?” These questions are easily addressed by students when they can use a stick figure (and their own body) for reference.
An example of this technique is shown in Scheme I. Initially, students may want to explicitly draw these stick figures. With practice, however, gaining the proper point of view within one’s mind becomes a simple, natural exercise. To draw the Fischer projection of a long, zigzag carbon chain (Scheme II), you must focus on the proper point of view for each chiral carbon, one carbon at a time. Since the groups on your left and right in the Fischer projection (OH and H) should always be sticking out toward you, view the carbon chain from the top (looking down) for carbons that point up on the zigzag (like C-2 and C-4) and view from the bottom (facing up) for the carbons that point down (such as C-3). I have found that this method can effectively help students interconvert line drawings and Fischer projections. More importantly, since there are no specific rules to follow, the visualization skills developed by using this stick figure technique can “stick” with these students long after they have completed the course. Acknowledgment I would like to thank Phil Beauchamp for his guidance in the preparation of this manuscript. W
Supplemental Material
A complete set of student instructions are available as a handout in this issue of JCE Online. Problem - convert this Fischer projection to a line drawing ? Cl ? CH 3
H
Cl CH3CH2
CH2CH3 A
CH3 CH3 head
Step 1 - initial orientation H
The CH3 is pointing up and away from me. The CH2CH3 is pointing down and away from me. The chlorine is pointing toward me and is on my right. The hydrogen is pointing toward me and is on my left.
Literature Cited 1. 2. 3. 4.
Cl CH2CH3 feet
5. 6. 7. 8.
Zhang, Q.-Z.; Zhang, S.-S. J. Chem. Educ. 1999, 76, 799. Mitschele, J. J. Chem. Educ. 1990, 67, 553. Argilés, J. M. J. Chem. Educ. 1986, 63, 927. Wheeler, D. M. S.; Wheeler, M. M., Wheeler, T. S. J. Chem. Educ. 1982, 59, 969. Signorella, S.; Sala, L. F. J. Chem. Educ. 1991, 68, 105. Mandal, D. K. J. Chem. Educ. 2000, 77, 866. Cossy, J.; Bellosta, V. J. Chem. Educ. 1998, 75, 1307. The following textbook makes use of a small cartoon person to view a stereocenter: Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry, 3rd ed.; Freeman: New York, 1998; p 180.
Step 2 - gaining the proper perspective
Correct point of view! The CH3 is pointing up and away from me. The CH3CH2 is pointing down (by my feet) and away from me. If I want the chlorine to be on my right, it must be on the wedged bond! Wrong POV! (vertical groups CH3 and CH3CH2 are sticking out at me)
Multiple chiral carbons - "view" one at a time Wrong POV! (CH3 is pointing down at my feet) Cl CH3CH2
The OH on C-4 is on my right H OH HO H
O
CH3 Wrong POV! (vertical groups CH3 and CH3CH2 are sticking out at me and CH3 is at my feet)
Scheme I: converting Fischer projections to line drawings
1486
The OH on C-2 is on my left
1
H
3
2
4
5
OH
HO H
The OH on C-3 is on my left
head CHO 1 HO
2
H
HO
3
H
H
4
OH
CH2OH 5 feet
Scheme II: converting line drawings to Fischer projections
Journal of Chemical Education • Vol. 78 No. 11 November 2001 • JChemEd.chem.wisc.edu
