Projection sawhorse

It is no accident that sections of our chaii cyclohexane drawings resemble sawhorse projections of staggered confonnations of alkanes. The same spatial relationships seen in alkanes cany over to substituents on a six-membered ring. In the structure... 

Sandmeyer reaction, 306 Sandwich compoimds, 275 Sawhorse projections, 7 Saytzev elimination, 249, 256 Schiff bases, 221 Schmidt rearrangement, 122 Selectivity, 156, 169, 326 a, 362 a, 370 372 aj,385 a bonds, 6 o complexes, 41,131 Sigmatropic rearrangements, 352-357 antarafacial, 353 carbon shifts, 354 hydrogen shifts, 352 orbital symmetry in, 352 photochemical, 354 suprafadal, 353 thermal, 353... 

Other common methods for representing the three-dimensional structures of molecules include Newman projections for showing conformational relationships and sawhorse figures. Newman projections look down a carbon-carbon bond so that the front carbon, designated by a circle, obscures the carbon directly behind it. Valences (bonds) to the front carbon extend to the center of the circle, while bonds to the rear carbon stop at the circle. Sawhorse projections have the carbon-carbon bond at oblique angles, which attempts to represent a perspective drawing of the molecule. Thus for 2-chloro butane, if one chooses to examine the 2,3 bond, then the sawhorse and Newman projections would be... 

Know the meaning of conformation, staggered, eclipsed, dash-wedge projection, Newman projection, sawhorse projection, rotational isomers, rotamers. 

Rotate around the C2-C3 bond to give an eclipsed sawhorse projection. This can be converted into a Fischer projection. Remember that vertical lines project behind the plane of the page and horizontal lines project out of the page. There are several correct Fischer projections in this case. The plane of symmetry, which is easily seen in the projection shown above, shows that this is meso-tartaric acid. 

Draw the structure of raeso-tartaric acid (2,3-dihydroxybutanedioic acid) with an anticlinal conformation in a sawhorse projection. 

Draw the formulae (sawhorse projection) of all the chiral conformations of 2-chloroethanol and name them. 

The first step is to convert the Fischer projection formulae into sawhorse projections. Note that for a base-induced elimination reaction of the E2 type, the tosyl group must be antiperiplanar to the proton or deutron being abstracted from the neighbouring carbon atom. As a consequence of this reaction mechanism, the configuration of the resulting double bond can be predicted. Because of the higher steric hindrance in the transition states leading to the Z isomers, in both cases the E isomer should be expected as the major product. 

The stereodescriptor erythro indicates that the chlorine atom and the hydroxy group lie on the same side of the main chain in a Fischer projection formula. Since it does not define an absolute configuration, two enantiomers must be considered. The Fischer projection formula, which represents an eclipsed conformation, is simplest first converted into a sawhorse projection (also in its eclipsed conformation) and then one side of the molecule rotated until both the reference groups, the chlorine atom and the hydroxy group, adopt an antiperiplanar arrangement. The required Newman projection formula can then be derived from these formulae. 

Considerations of minimum overlap of radii of nonbonded substituents on the polymer chain are useful in understanding the preferred conformations of macromolecules in crystallites. The simplest example for our purposes is the polyethylene (1-3) chain in which the energy barriers to rotation can be expected to be similar to those in /i-butane. Figure 4-2 shows sawhorse projections of the conformational isomers of two adjacent carbon atoms in the polyethylene chain and the corresponding rotational energy barriers (not to scale). The angle of rotation is that between the polymer chain substitutents and is taken here to be zero when the two chain segments are as far as possible from each other. 

A sawhorse representation and a Newman projection of ethane. The sawhorse projection views the molecule from an oblique angle, while the Newman projection views the molecule end-on. 

Around each carbon the bonds are still arranged in a tetrahedral fashion. Now redraw this using the sawhorse projection. 

As a prototype molecule, tartaric acid (2 2,3-dihydroxybutanedioic acid), shown here without stereochemical specification, is chosen. Tartaric acid is a compound of considerable historical importance (Section 3.6), and derivatives of tartaric acid are still compounds of contemporary research interest (see Seebach et al.1). (2i ,3i )-(+)-Tartaric acid is shown in sawhorse projection in 3. 

Give a stereo drawing or sawhorse projection of the rneso stereoisomer of 78. 

A meso compound is represented by, for example, (l/ ,25), and the identical (15,2/ ) structure is not shown. In addition, only one enantiomer of a pair is shown. Where relevant, sawhorse projections are shown in eclipsed conformation, though it should be recognized that this is not the most stable. 

Sawhorse projections reveal another reason that anti ehmination is favored. In syn eliminahon, the electrons of the departing hydrogen move to the front side of the carbon bonded to X, whereas in anti elimination, the electrons move to the back side of the carbon bonded to X. We have seen that displacement reactions involve back-side attack because the best overlap of the interacting orbitals is achieved through back-side attack (Section 10.2). Finally, anti elimination avoids the repulsion the electron-rich base experiences when it is on the same side of the molecule as the electron-rich departing halide ion. 

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