Newman projections, and

Among the various ways in which the staggered and eclipsed forms are portrayed, wedge-and-dash, sawhorse, and Newman projection drawings are especially useful. These are shown for the staggered conformation of ethane in Figure 3.2 and for the eclipsed conformation in Figure 3.3. 

The above quasi three-dimensional representations are known as sawhorse and Newman projections, respectively. The eclipsed and staggered forms, and the infinite variety of possible structures lying between them as extremes, are known as conformations of the ethane molecule conformations being defined as different arrangements of the same group of atoms that can be converted into one another without the breaking of any bonds. 

FIGURE 22. Top Labels of the four localized basis w-orbitals of [4.4]spirononatetraene 247 and Newman projection defining their relative phases. Middle Newman projections of the four linear combinations ld2(tr), b <7T) and 1 c(jt). defined in equation 59. Bottom Correlation diagram showing the splitting due to spiroconjugation between the butadiene moieties in 247... 

Problem 7.34 Assuming that anti elimination is favored, illustrate the stereospecificity of the E2 dehydro-halogenation by predicting the products formed from (a) tneso- and (b) either of the enantiomers of 2,3-dibromobutane. Use the wedge-sawhorse and Newman projections. -4... 

FIGURE 1. Conformations of cyclopropyl derivatives, characterized by dihedral angle t(M—Cl—C4—X) perspective views (above) and Newman projections down theC4—Cl bond (below). M is the midpoint of the C2—C3 bond... 

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... 

TABLE 9. Conformational structures (a) and Newman projection diagrams (b) of (IS,25)- and (IR,2ft)-cyclohexane diol. Bis-p-dimethylaminobenzoate derivatives (c), bisignate CD Cotton effect data and torsion angles (d)14... 

Fig. 2. Dreiding models and Newman projections of the terpenic unit present in pauridianthoside (26) (A) and in isopauridianthoside (27) (B).

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. 

Table XI. Conformational structures (a and b) and Newman projection diagrams (c and d) of (15,25) and (lR,2R)-cyclohexane diol (a) and their bis-p-dimethylaminobenzoate derivatives (b). CD data for the bisignate Cotton effects of the latter are shown below.

The two chair conformations of methylcyclohexane interconvert at room temperature, so the one that is lower in energy predominates. Careful measurements have shown that the chair with the methyl group in an equatorial position is the most stable conformation. It is about 7.6 kJ/mol (1.8 kcal/mol) lower in energy than the conformation with the methyl group in an axial position. Both of these chair conformations are lower in energy than any boat conformation. We can show how the 7.6 kJ energy difference between the axial and equatorial positions arises by examining molecular models and Newman projections of the two conformations. First, make a model of methylcyclohexane and use it to follow this discussion. 

New problems show the student how to interconvert and name structural formulas and Newman projections. 

In the second method we draw the end-on view, looking along the C-C bond. This view is known as a Newman projection, and Newman projections are subject to a few conventions e The carbon atom nearer the viewer is at the junction of the front three bonds The carbon further away (which can t in fact be seen in the end-on view) is represented by a large circle. This makes the perspective inaccurate—but this doesn t matter... 

FIGURE 1. Bond scheme (a) and Newman projections of the enamine grouping down N1—C2 (b) and C2—N1 (c) directions based on Brown et al.1 The symbols are those used throughout this chapter... 

FIGURE 12.7. Fischer and Newman projections of enantiomers (mirror images of each other). One enantiomer is on the left and the other is on the right, divided by a vertical line (a mirror perpendicular to the plane of the page), (a) Fischer diagram, and (b) Newman projection. 

In the second method we draw the end-on view, looking along the C-C bond. This view is known as a Newman projection, and Newman projections are subject to a few conventions ... 

A 3D model of the receptor binding site has been worked out with the aid of conformationally restrained analogues of acetylcholine. Acetylcholine itself has no conformational restraints. It is a straight-chain molecule in which bond rotation along the length of its chain can lead to numerous possible conformations (or shapes). Thus, it is impossible to know exactly the 3D shape of the receptor site from considering acetylcholine alone. In the past, it was assumed that a flexible neurotransmitter such as acetylcholine would interact with its receptor in its most stable conformation. In the case of acetylcholine, that would be the conformation represented by the sawhorse and Newman projections shown in Fig. 11.12. 

FIGURE 1. Bond scheme (a) and Newman projections of the enamine grouping down N —... 

Fig. 14.4 (a) The structure of N2H4, and Newman projections showing (b) the observed gauche conformation, and (c) the possible staggered conformation. An eclipsed conformation is also possible. 

Draw perspective formulas and Newman projections for the eclipsed and staggered forms of propane. 

In the eclipsed form of propane there is more steric hindrance than in the staggered form. There is also a greater torsional strain in the eclipsed form. As a result, the eclipsed form is expected to be higher in energy than the staggered form. Therefore the staggered conformation is more stable. The perspective formulas and Newman projections are sketched below ... 

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