Fischer projections stereocenters

There is an entirely different way to draw stereocenters (instead of using regular bond-line drawings with dashes and wedges). Fischer projections are helpful for drawing molecules that have many stereocenters, one after another. These drawings look like this ... 

You might be wondering how you would determine the conhguration of a stereocenter when you are given a Fischer projection. If each stereocenter is drawn as two wedges and two dashes, how do you figure out how to look at the stereocenter The answer is simple. Just choose one dash and one wedge, and draw them... 

Fischer projections can also be used for compounds with just one stereocenter, as above, but they are usually used to show compounds with multiple stereocenters. You will utilize Fischer projection heavily when you learn about carbohydrates at the end of the course. 

Now we can understand why we cannot draw a Fischer projection sideways. If we did, we would be inverting the stereocenter. To draw the enantiomer of a Fischer projection, do not turn the drawing sideways. Instead, you should use the second method we saw for drawing enantiomers (place the mirror on the side of the compound and draw the reflection). Recall that this was the method that we used for drawings where wedges and dashes were implied but not shown. Fischer projections are another example of drawings that fit this criterion ... 

Answer We begin by drawing the stereocenter as it is implied by the Fischer projection ... 

Erythro/threo Terms derived from carbohydrate nomenclature used to describe the relative configuration at adjacent stereocenters. Erythro refers to a configuration with identical or similar substituents on the same side of the vertical chain in Fischer projection. Conversely, a threo isomer has these substituents on opposite sides. These terms came from the nomenclature of two carbohydrate compounds, threose and erythrose (see Fig. 1-35). 

The Fischer projections show that isotactic placement corresponds to meso or m-place-ment for a pair of consecutive stereocenters. Syndiotactic placement corresponds to racemo (for racemic) or r-placement for a pair of consecutive stereocenters. The configurational... 

The designations D and L refer to the configuration of the stereocenter farthest from the aldehyde or ketone group on a monosaccharide chain. When the monosaccharide chain is drawn as a Fischer projection, a D-monosaccharide has the -OH on this carbon on the right an L monosaccharide has it on the left. 

D-Sugars and L-Sugars When glyceraldehyde is drawn as a Fischer projection, the OH group on the stereocenter points to the right (Figure 12.6). 

Fischer-Rosanoff convention A method for the specification of absolute configuration, still in common use for amino acids and sugars. When drawn in a Fischer projection with Ci at the top, if the functional group of the specified stereocenter is on the right, the absolute configuration is D, if on the left, it is L. For amino acids, the reference stereocenter is C2 for sugars it is the highest numbered stereocenter [98],... 

The relative configuration of adjacent substituents in a Fischer projection formula are designated erythro if they are on the same side and threo if they are on the opposite side. The stereochemistry of adjacent stereocenters can also be usefully represented... 

Assign configuration, using the sequence rule, to each stereocenter in the stereoisomers citric acids shown below and convert the Fischer projections to extended chain representations. 

Many organic compounds have more than one asymmetric carbon. The more asymmetric carbons a compound has, the more stereoisomers are possible for the compound. If we know how many asymmetric carbons a compound has, we can calculate the maximum number of stereoisomers for that compound a compound can have a maximum of 2 stereoisomers (provided it doesn t have any other stereocenters), where n equals the number of asymmetric carbons. For example, 3-chloro-2-butanol has two asymmetric carbons. Therefore, it can have as many as four (2 = 4) stereoisomers. The four stereoisomers are shown both as perspective formulas and as Fischer projections. 

Problem 18.54. In the Fischer projection (Figs. 18-1 and 18-2), o-carbohydrates are those where the OH on the stereocenter furthest from the C = O is placed on the right-hand side. What is common to all o-carbohydrates in the Haworth structures ... 

In addition to the different bond orders, there are several other types of bonds to handle stereochemistry (bold, dotted and wavy bonds). Stereocenters are displayed by showing the respective atom in reverse video. For molecules with many stereocenters (e.g. sugar chemistry), the Fischer Projection Method can be used for entering stereocenters. (Figures 6,7)... 

Fischer projection A two-dimensional representation showing the configuration of a stereocenter horizontal lines represent bonds projecting forward from the stereocenter, whereas vertical lines represent bonds projecting to the rear. 

Chemists commonly use two-dimensional representations called Fischer projections to show the configuration of carbohydrates. To draw a Fischer projection, draw a three-dimensional representation with the most oxidized carbon toward the top and the molecule oriented so that the vertical bonds from the stereocenter are directed away from you and the horizontal bonds from it are directed toward you. Then write the molecule as a two-dimensional figure with the stereocenter indicated by the point at which the bonds cross. You now have a Fischer projection. 

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