Chiral molecules Fischer projections

Carbohydrates Chiral Molecules Fischer Projections of Monosaccharides Haworth Structures of Monosaccharides Chemical Properties of Monosaccharides Disaccharides Polysaccharides... 

Lei s relurn fo bromochlorofluoromelhane as a simple example of a chiral mole cule The Iwo enanliomers of BrClFCH are shown as ball and slick models as wedge and dash drawings and as Fischer projections m Figure 7 6 Fischer projeclions are always generated Ihe same way Ihe molecule is oriented so lhal Ihe verlical bonds al Ihe chiralily center are directed away from you and Ihe horizonlal bonds poinl toward you A projeclion of Ihe bonds onto Ihe page is a cross The chiralily center lies al Ihe center of Ihe cross bul is nol explicilly shown... 

Erythro (Section 7 11) Term applied to the relative configura tion of two chirality centers within a molecule The erythro stereoisomer has like substituents on the same side of a Fischer projection... 

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. 

Because a given chiral molecule can be drawn in many different ways, it s often necessary to compare two projections to see if they represent the same or different enantiomers. To test for identity, Fischer projections can be moved around on the paper, but only two kinds of motions are allowed moving a Fischer projection in any other way inverts its meaning. 

Problem 25.4 Redraw the following molecule as a Fischer projection, and assign R or 5 configuration to the chirality center (yellow-green = Cl) ... 

Absolute configuration (Section 9.5) The exact three-dimensional structure of a chiral molecule. Absolute configurations are specified verbally by the Cahn-Ingold-Prelog R,S convention and are represented on paper by Fischer projections. 

Fischer projection (Section 25.2) A means of depicting the absolute configuration of a chiral molecule on a flat page. A Fischer projection uses a cross to represent the chirality center. The horizontal arms of the cross represent bonds coming out of the plane of the page, and the vertical arms of the cross represent bonds going back into the plane of the page. 

With these restrictions Fischer projections may be used instead of models to test whether a molecule containing asymmetric carbons is superimposable on its mirror image. However, there are no such conventions for molecules whose chirality arises from anything other than chiral atoms when such molecules are examined on paper, three-dimensional pictures must be used. With models or three-dimensional pictures there are no restrictions about the plane of the paper. 

Having identified 6 centres of chirality, i.e. 4 due to the side-chains and 2 due to the atropisomerism, a total of 64 optical isomers are possible within each of the rotamers Gl, G2, G3. However, this number is reduced to 16 configurations because of identities which can be visualized as follows The Fischer projections of the iotrolan molecule are split at the bridge into two halves to give the... 

Fischer projections provide a fnrther approach to the two-dimensional representations of three-dimensional formulae. They become particularly useful for molecules that contain several chiral centres, and are most frequently encountered in discussions of sugars (see... 

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. 

Fischer projection (Section 7.8) A two-dimensional drawing of a chiral molecule in which the chirality center is represented as a cross with the atom at its coiter. Although the four bonds to the chirality center are shown in the plane of the page, the horizontal bonds project above the plane of the page and the vertical bonds project behind the page. 

Mirror planes of symmetry are particularly easy to identify from the Fischer projection because this projection is normally the most symmetric conformation. In the first preceding example (propan-2-ol) and in the following example [(2S,3f )-2,3-dibromobutane], the symmetry planes are indicated in red these molecules with symmetry planes cannot be chiral. 

---