Horizontal Fischer projection

If you view the second formula from the top, you will see that it is just a three-dimensional representation of the Fischer projection. Horizontal groups at each stereogenic center come up toward you, and vertical groups recede away from you. The second formula represents an eclipsed conformation of D-threose. The third and fourth formulas represent sawhorse and Newman projections, respectively, of a staggered conformation of D-threose. 

In a Fischer projection horizontal lines project out of the plane of the paper toward the viewer and vertical lines extend back from the plane of the paper away from the viewer. 

What happens when we rotate a Fischer projection in the plane of the page by 90° Does the result depict the spatial atraugement of the original molecule The definition of a Fischer projection—horizontal bonds are pointed above, vertical ones below the plane of the page—tells us that the answer is clearly no, because this rotation has switched the relative spatial disposition of the two sets The result is a picture of the enantiomer. On the other hand, rotation by 180° is fine, because horizontal and vertical lines have not been interchanged The resulting drawing represents the same enantiomer. 

Section 7 7 A Fischer projection shows how a molecule would look if its bonds were projected onto a flat surface Horizontal lines represent bonds coming toward you vertical bonds point away from you The projection is nor mally drawn so that the carbon chain is vertical with the lowest num bered carbon at the top... 

The aldotetroses are the four stereoisomers of 2 3 4 trihydroxybutanal Fischer pro jections are constructed by orienting the molecule m an eclipsed conformation with the aldehyde group at the top The four carbon atoms define the mam chain of the Fischer projection and are arranged vertically Horizontal bonds are directed outward vertical bonds back... 

In the Fischer convention, the ermfigurations of other molecules are described by the descriptors d and L, which are assigned comparison with the reference molecule glyceraldehyde. In ertqrloying the Fischer convention, it is convenient to use projection formulas. These are planar representations defined in such a w as to convey three-dimensional structural information. The molecule is oriented with the major carbon chain aligned vertically in such a marmer that the most oxidized terminal carbon is at the top. The vertical bonds at each carbon are directed back, away fiom the viewer, and the horizontal bonds are directed toward the viewer. The D and L forms of glyceraldehyde are shown below with the equivalent Fischer projection formulas. 

Fischer projection formulas can be used to represent molecules with several stereogenic centers and are commonly used for caibohydrates. For other types of structures, a more common practice is to draw the molecule in an extended conformation witii the main chain horizontal. In this arrangement, each tetrahedral caibon has two additional substituents, one facing out and one in. The orientation is specified widi solid wedged bonds for substituents facing out and with dashed bonds for substituents that point in. 

Let s return to bromochlorofluoromethane as a simple example of a chiral molecule. The two enantiomers of BrCIFCH are shown as ball-and-stick models, as wedge-and-dash drawings, and as Fischer projections in Figure 7.6. Fischer projections are always generated the same way the molecule is oriented so that the vertical bonds at the chirality center are directed away from you and the horizontal bonds point toward you. A projection of the bonds onto the page is a cross. The chirality center lies at the center of the cross but is not explicitly shown. 

A tetrahedral carbon atom is represented in a Fischer projection by two crossed lines. The horizontal lines represent bonds coming out of the page, and the vertical lines represent bonds going into the page. 

For a thorough understanding of stereochemistry, it is useful to examine molecular models (like those depicted in Fig. 4.1). However, this is not feasible when writing on paper or a blackboard. In 1891, Emil Fischer greatly served the interests of chemistry by inventing the Fischer projection, a method of representing tetrahedral carbon atoms on paper. By this convention, the model is held so that the two bonds in front of the paper are horizontal and those behind the paper are vertical. 

This is our Fischer projection. All of the configurations can be seen on this drawing, because we are able to picture in our minds what the 3D shape is. So the rule is that all horizontal lines are coming out at us, and all vertical lines are going away from us ... 

In a Fischer projection of representing an optically active compound, an asymmetric atom is shown at the intersection point of two lines of a cross. The horizontal lines going to left and right of this point are the bonds extending forward from the plane of paper. The two vertical lines going to top and bottom are those going back away from the plane of paper. For example 2-iodobutane can be represented as ... 

Fischer polypeptide synthesis org chem A synthesis of peptides in which a-amino acids or those peptides with a free amino group react with acid halides of a-haloacids, followed by amination with ammonia. fish-ar pal-e pep,tTd. sin tha sas ) Fischer projection orgchem) A method for representing the spatial arrangement of groups around chiral carbon atoms the four bonds to the chiral carbon are represented by a cross, with the assumption that the horizontal bonds project toward the viewer and the vertical bonds away from the viewer fish-ar pra.jek-shon) Fischer s salt See cobalt potassium nitrite. fish-3rz solt)... 

It is also surprisingly easy to assign R ov S configurations to chiral carbons in the Fischer projections but, because horizontal lines imply wedged bonds (towards you) and vertical lines imply dotted bonds (away from you), there are important guidelines to remember ... 

We can draw these three stereoisomers as Fischer projections, reversing the configurations at both centres to get the enantiomeric stereoisomers, whilst the Fischer projection for the third isomer, the meso compound, is characterized immediately by a plane of symmetry. For (-l-)-tartaric acid, the configuration is 2R, >R), and for (—)-tartaric acid it is (2S,3S). For both chiral centres, the group of lowest priority is hydrogen, which is on a horizontal line. In fact, this is the case in almost all Fischer projections, since, by convention, the vertical... 

Throughout this document, stereochemical formulae for polymer chains are shown as Fischer projections rotated through 90°, i.e., displayed horizontally rather than vertically,... 

The use of rotated Fischer projections corresponds to the practice of using horizontal lines to denote polymer backbone bonds, but it is most important to note that this does not give an immediately visual impression of the zigzag chain. In the projections as used in this document, at each individual backbone carbon atom the horizontal lines represent bonds directed below the plane of the paper from the carbon atom while the vertical lines project above the plane of the paper from the carbon atom. Thus, the rotated Fiseher projection... 

The pioneering work of Natta and co-workers introduced the concept of tacticity, i.e. the orderliness of the succession of configurational repeating units in the main chain of a polymer. For example, in polypropene (polypropylene), possible steric arrangements are (shown in Fischer projections displayed horizontally) ... 

An isotactic polymer has only one species of configurational base unit in a single sequential arrangement and a syndiotactic polymer shows an alternation of configurational base units that are enantiomeric, whereas in an atactic polymer the molecules have equal numbers of the possible configurational base units in a random sequence distribution. This can be generalized as follows in zig-zag and horizontal Fischer projections ... 

The best way to see if the configuration has changed is to determine how many swaps of groups must be made to go from the initial to the final Fischer projection. An even number of swaps leaves the configuration unchanged, while an odd number results in a changed configuration. Convince yourself that to get (/) takes three swaps, while to get (ii) takes two swaps (the horizontal pairs and the vertical pairs). 

FIGURE 7-2 Three ways to represent the two stereoisomers of glyc-eraldehyde. The stereoisomers are mirror images of each other. Ball-and-stick models show the actual configuration of molecules. By convention, in Fischer projection formulas, horizontal bonds project out of the plane of the paper, toward the reader vertical bonds project behind the plane of the paper, away from the reader. Recall (see Fig. 1-17) that in perspective formulas, solid wedge-shaped bonds point toward the reader, dashed wedges point away. 

In writing Fischer projections of molecules with two chirality centers, the molecule is arranged in an eclipsed conformation for projection onto the page, as shown in Figure 7.9. Again, horizontal lines in the projection represent bonds coming toward you vertical bonds point away. 

An early method to picture the three-dimensional properties of molecules was the use of Fischer projections. In Fisher projections, bonds are drawn either vertically or horizontally. Bonds which are vertical project into file space behind the plane of file paper (blackboard, computer screen). Bonds which are horizontal project into file space in front of file plane of the paper (blackboard, computer screen). 

The Fischer projection is also important on the MCAT- hi Fischer projections the vertical lines are assumed to be oriented into the page. The horizontal lines are assumed to be oriented out of the page. 

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