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Understanding Fischer Projection and Angular Line Representation Conversion Luis F. Moreno* Grupo Metodología de la Ense~ nanza de la Química, Grupo de Química-Física Teorica, Instituto de Química, Universidad de Antioquia, AA 1226 Medellín, Colombia ABSTRACT: Difficulty for undergraduate students taking a basic organic chemistry course arises when they have to understand the relationship between the different molecular representations, for example, between Fischer and angular line representations. It is well known that some techniques describe the conversion between the Fisher and Howarth projections and others describe the conversion between the Fischer and angular line projections. This text offers an easy way to understand the conversion between the Fischer projection and angular line representation, emphasizing the structural relationship of the two models. KEYWORDS: Second-Year Undergraduate, Organic Chemistry, Analogies/Transfer, Chirality/Optical Activity, Conformational Analysis
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ifficulty for undergraduate students taking a basic organic chemistry course arises when they have to understand the relationship between the different molecular representations, for example, between Fischer and angular line representations. Some techniques to describe the conversion between the Fisher and Haworth projections have been published,1 4 and other techniques to describe the conversion between the Fischer and angular line projections have been published as well.5 8 This text offers an easy way to understand the conversion between the Fischer projection and angular line representation, emphasizing the structural relationship of the two models. Fischer projections are widely used in the representation of systems with a large number of chiral centers. A simplified carbohydrate model with a single chiral center (A) is used to describe a method to understand the conversion. The method involves a 90° rotation of a Fischer projection to the right, around a vertical axis in the plane of the page (B), followed by a second 90° rotation in a counterclockwise direction around an axis perpendicular to the plane of the page, thus, converting a Fischer projection into an angular line projection (C). Figure 1 shows the conversion and could easily be used in conjunction with handheld molecular models to further illustrate the conversion. An interesting feature in a Fischer projection of a molecule that contains multiple chiral centers (D) is that all the carbons in the chain acquire a completely eclipsed conformation (Figure 2). A 90° rotation to the right around a vertical axis in the plane of the page (E) illustrates this point. The Fischer projection is an angular line, flat, cyclic representation, where all the main chain carbons are completely eclipsed, this being the highest-energy conformer. The angular line conformation (F) results in the lowest-energy representation, where all the main chain carbons are in a staggered conformation in the classical representation of a zigzag. The general instructions to convert from a Fischer representation to the angular line representation are as follows: • The Fischer projection performs a 90° rotation on a vertical axis in the plane of the page. If the rotation is done to the Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
Figure 1. Conversion between Fischer projection and angular line representation for a compound with one chiral center.
Figure 2. Conversion between Fischer projection and angular line representation for a compound with two chiral centers; eclipsed and anti Newman projections are shown below the angular line representations.
right, all the substituents located on the left side out of the plane of the page are now located on bold wedges out of the plane, and the substituents located on the right side entering the plane of the page are on dashed-wedges. The carbon chain stays in the plane similar to open cyclic representation (compare D and E in Figure 2). • Draw a zigzag chain of an equal number of carbon atom to the central chain of the Fischer projection and the spatial relationship of the two substituents on the first chiral carbon will be the same as when the Fischer projection was rotated. Published: November 01, 2011 175
dx.doi.org/10.1021/ed101011c | J. Chem. Educ. 2012, 89, 175–176
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it would be reversed in a Fischer projection (L). C4 has a hydroxyl group that is separated from C2 by C3, then the relationship between the two hydroxyl groups will be the same in a Fischer projection; likewise. the relationship between the substituents on C4 and C5 will be reversed in a Fischer projection.
’ AUTHOR INFORMATION Figure 3. The compound (2S,4R,5S)-2-bromo-4-chloro-5-hydroxy-3hexanone; conversion between Fischer projection and angular line representation.
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’ ACKNOWLEDGMENT I would like to thank Wilber Silva and Elizabeth Escobar for their guidance in the preparation of this manuscript. ’ REFERENCES (1) Argiles, J. M. J. Chem. Educ. 1986, 63, 927. (2) Mitschele, J. J. Chem. Educ. 1990, 67, 553. (3) Zhang, Q.-Z.; Zhang, S.-S. J. Chem. Educ. 1999, 76, 799. (4) Wheeler, D. M. S.; Wheeler, M. M.; Wheeler, T. S. J. Chem. Educ. 1982, 59, 969. (5) Starkey, Laurie S. J. Chem. Educ. 2001, 78, 1486. (6) Signorella, S.; Sala, L. F. J. Chem. Educ. 1991, 68, 105. (7) Mandal, D. K. J. Chem. Educ. 2000, 77, 866. (8) Clayden, J.; Greeves, N.; Warren, S.; Wothers, P. Organic Chemistry, 1st ed.; Oxford University Press: New York, 2001; p 395.
Figure 4. Conversion between angular line representation and Fischer projection for glucose.
The position of other substituents depends on whether they are on the same side or the opposite side in the Fischer projection (compare E and F in Figure 2). The following guidelines help identify the type of spatial relationship between the substituents upon the conversion: • The relationship between substituents on adjacent atoms in a Fischer projection will be reversed in the angular line representation. • The relationship between two substituents on atoms separated by one atom in a Fischer projection will be the same in the angular line representation.
’ EXAMPLES In (2S,4R,5S)-2-bromo-4-chloro-5-hydroxy-3-hexanone (Figure 3), C2 has a bromide substituent on the right side in the Fischer projection (G). This group is represented as a dashed bond behind the plane in angular line form (H). Note that the carbonyl group is located in the plane perpendicular to the page in the Fischer projection, shown in G on the right side. They are shown in the rotated Fischer projection and open-chain cyclic structure (H); the chlorine and bromine are located in the same side in the Fischer projection, and because they are separated from one another by the carbonyl carbon, the chlorine and bromine will end up on the same side in the zigzag angular line drawing (I) as dashed bonds behind the plane. Finally, the relationship between substituents chlorine and hydroxyl on adjacent atoms C4 and C5, respectively, in a Fischer projection will be reversed in the zigzag angular line representation. As a final example of using this method, the representation of the conversion for glucose (Figure 4) begins by drawing an open cyclic chain (K) of an equal number of carbon atoms to the zigzag angular line representation (J). In J, C2 has a hydroxyl group represented as a dashed bond behind the plane and must remain the same in K; C3 has a hydroxyl group as a dashed bond too, and 176
dx.doi.org/10.1021/ed101011c |J. Chem. Educ. 2012, 89, 175–176
