Fischer Configuration

[James Gillock]

In chemistry, the Fischer projection, devised by Emil Fischer in 1891, is a two-dimensional representation of a three-dimensional organic molecule by projection. Fischer projections were originally proposed for the depiction of carbohydrates and used by chemists, particularly in organic chemistry and biochemistry. The use of Fischer projections in non-carbohydrates is discouraged, as such drawings are ambiguous and easily confused with other types of drawing. The main purpose of Fischer projections is to show the chirality of a molecule and to distinguish between a pair of enantiomers. Some notable uses include drawing sugars and depicting isomers.[1]

All bonds are depicted as horizontal or vertical lines. The carbon chain is depicted vertically, with carbon atoms sometimes not shown and represented by the center of crossing lines (see figure below). The orientation of the carbon chain is so that the first carbon (C1) is at the top.[2] In an aldose, C1 is the carbon of the aldehyde group; in a ketose, C1 is the carbon closest to the ketone group, which is typically found at C2.[3]

The proper way to view a Fischer projection is to vertically orient the molecule in relation to the carbon chain, have all horizontal bonds point toward the viewer, and orient all vertical bonds to point away from the viewer.[4] Molecules with a simple tetrahedral geometry can be easily rotated in space so that this condition is met (see figures). Fischer projections are commonly constructed beginning with a sawhorse representation. To do so, all attachments to main chain carbons must be rotated such that resulting Newman projections show an eclipsed configuration.[2] The carbon chain is then positioned vertically upward with all horizontal attachments pointing toward the viewer.[2] Finally, attachments to main chain carbons that face away from the viewer are placed in the vertical position of the Fischer projection, and those that face toward the viewer are placed in the horizontal position of the Fischer projection.[4] Each intersection between a horizontal and vertical line on the Fischer projection represents a carbon in the main carbon chain.[2]

Fischer projections are effective representations of 3D molecular configuration in certain cases. For example, a monosaccharide with three carbon atoms (triose), such as the D-Glyceraldehyde depicted above, has a tetrahedral geometry, with C2 at its center, and can be rotated in space so that the carbon chain is vertical with C1 at the top, and the horizontal bonds connecting C2 with the Hydrogen and the Hydroxide are both slanted toward the viewer.

However, when creating a Fischer projection for a monosaccharide with more than three carbons, there is no way to orient the molecule in space so that all horizontal bonds will be slanted toward the viewer. After rotating the molecule so that both the horizontal bonds with C2 are slanted toward the viewer, the horizontal bonds with C3 will be typically slanted away. So, after drawing the bonds with C2, before drawing the bonds with C3 the molecule must be rotated in space by 180 about its vertical axis. Further similar rotations may be needed to complete the drawing.

This implies that in most cases a Fischer projection is not an accurate representation of the actual 3D configuration of a molecule. It can be regarded as a projection of a modified version of the molecule, ideally twisted at multiple levels along its backbone. For instance, an open-chain molecule of D-glucose rotated so that the horizontal bonds with C2 are slanted toward the viewer, would have the bonds with C3 and C5 slanted away from the viewer, and hence its accurate projection would not coincide with a Fischer projection. For a more accurate representation of an open-chain molecule, a Natta projection may be used.

According to IUPAC rules, all hydrogen atoms should preferably be drawn explicitly; in particular, the hydrogen atoms of the end group of carbohydrates should be present.[5] In this regard Fischer projection is different from skeletal formulae.

Haworth projections are a related chemical notation used to represent sugars in ring form. The groups on the right hand side of a Fischer projection are equivalent to those below the plane of the ring in Haworth projections.[9] Fischer projections should not be confused with Lewis structures, which do not contain any information about three dimensional geometry. Newman projections are another system that can be used as they showcase the structure of a molecule in the staggered or eclipsed conformation states.[10] The wedge and dash notation will help to showcase the stereochemistry within a specific molecule.

Recall that the Cahn-Ingold-Prelog (CIP) rules for determining R and S configurations outline a set of rules for assigning priorities (1, 2, 3, and 4) to each of the groups assigned to a chiral center.

For a sugar drawn in a Fischer projection with the most oxidized group at the top (i.e. a carboxylic acid or aldehyde), a chiral center with OH on the right will be R, and a chiral center with OH on the left will be S.

hello sir.
could you please determine the prioritize of the structure in the link? for example CD3 is the first priority or 14CH3 is the first? please arrange them for me. tnx
i asked this problem to some ppl. but they answered me different .
link of the image:
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This gives us the following shortcut. If a group is on the left side of the Fischer, it will end up on the top face of the Haworth, and if it is on the right side of the Fischer, it will end up on the bottom face of the Haworth.

galactose configuration is wrong I suppose.
D-galactose can be obtained by flipping configuration at C-4 for D-glucose. Now to convert D to L, configuration at C-5 is flipped too. This results in a structure, where -OH groups point to the left on C-3, C-4 and C-5

A Fischer projection or Fischer projection formula is a convention used to depict a stereoformula in two dimension without destroying the stereochemical information, i.e., absolute configuration, at chiral centers.

A Fischer projection restricts a three-dimensional molecule into two dimensions. Consequently, there are limitations as to the operations that can be performed on a Fischer projection without changing the absolute configuration at chiral centers. The operations that do not change the absolute configuration at a chiral center in a Fischer projections can be summarized as two rules.

The above rules assume that the Fischer projection under consideration contains only one chiral center. However, with care, they can be applied to Fischer projections containing any number of chiral centers.

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Fischer projections are just another way of drawing compounds contacting chirality centers. They were initially proposed by Emil Fischer for making it easier to draw the structures of compounds containing multiple chirality centers with the main idea of not having to draw the wedge and dash lines for every single chiral center. This is especially applicable and used mostly for drawing sugars.

Before getting to drawing its Fischer projection, lets number the carbons in any order (no IUPAC rules needed). Remember, numbering carbons will always be helpful no matter what you need to do with an organic structure.

If you look at the molecule from the top, you will see the following representation where the two groups on the side are pointing towards and the ones on the top and on the bottom are pointing away from you. We will show the ones on the sides with wedge lines and the others with dashed lines:

There are two wedge and two dash lines which may look strange to you since we always have one of each and then the two solid lines but it is okay-it all depends on the direction we are looking at the molecule.

This, however, is not the Fischer projection yet, since, remember, we said the main idea was to avoid showing wedge and dash lines yet being able to convey absolute configuration of the chirality centers (R, S).

For this, we are going to draw the molecule and simply show all the bonds with plane solid lines, keeping in mind that the horizontal groups are pointing towards you and the ones on the vertical line are pointing away from you:

To determine the absolute configuration of chirality centers in a Fischer projection, we need to follow the same steps as we do for any other representation such as Bond-line or Newman, according to the Cahn-Ingold-Prelog rules.

The arrow goes clockwise which indicates R configuration. However, this is where you need to remember that the horizontal groups (Cl and H) are pointing towards you, therefore, the configuration must be switched from R to S. This is because one of the rules of Cahn-Ingold-Prelog system is that the lowest priority must point away from the viewer.

Fischer projections make it easy to draw different stereoisomers. As an example, if you are asked to draw the enantiomer of the following molecule with three chiral centers, you can draw an imaginary mirror plane and draw the reflection of the molecule, which is achieved simply by swapping the two group of a chiral center:

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