Newman projections conformations

Figure 3 Twist-envelope conformations (Newman projection down C—C bond) for 1,3,2-dioxathiolane 2-...

Scheme 4 Twist-envelope conformations (Newman projection down C—C bond) and H NMR data for the ring hydrogens of 1,3,2-dioxathiolane 2-oxide (16) and its cis (23) and trans (24) methyl derivatives...

CH2—CHR-, and - CH2—CHR—O-. Without going any further into individual types of conformations at this stage, it need only be mentioned that the Fischer projection corresponds to an eclipsed, and the Natta projection to a staggered conformation. Newman projections can represent both eclipsed and staggered conformations. As a rule, trans-staggered and as-eclipsed conformations are used as the basis of such projections. (For the significance of these expressions see Section 4.5.)... 

FIGURE 3 6 The gauche and anti conformations of butane shown as ball and spoke mod els left) and as Newman projections right) The gauche conformation is less stable than the anti because of the van der Waals strain between the methyl groups... 

The stereoselectivity of elimination of 5 bromononane on treatment with potassium ethox ide was described in Section 5 14 Draw Newman projections or make molecular models of 5 bromononane showing the conformations that lead to cis 4 nonene and trans 4 nonene respec tively Identify the proton that is lost in each case and suggest a mechanistic explanation for the observed stereoselectivity... 

Acyclic Compounds. Different conformations of acyclic compounds are best viewed by construction of ball-and-stick molecules or by use of Newman projections (see Fig. 1.2). Both types of representations are shown for ethane. Atoms or groups that are attached at opposite ends of a single bond should be viewed along the bond axis. If two atoms or groups attached at opposite ends of the bond appear one directly behind the other, these atoms or groups are described as eclipsed. That portion of the molecule is described as being in the eclipsed conformation. If not eclipsed, the atoms... 

Vicinal CH coupling constants Hqh resemble vicinal HH coupling constants in the way that they depend on the cosine of the dihedral angle 9 between the CC bond to the coupled C atom and the C//bond to the coupled proton (cf Fig. 2.16), as illustrated by the Newman projections of the conformers 20a-c of a propane fragment. 

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. 

Higher alkanes having unbranched carbon chains are, like butane, most stable in theh all-anti conformations. The energy difference between gauche and anti conformations is similar- to that of butane, and appreciable quantities of the gauche conformation are present in liquid alkanes at 25°C. In depicting the conformations of higher alkanes it is often more helpful to look at them from the side rather than end-on as in a Newman projection. Viewed from this perspective, the most stable conformations of pentane and hexane... 

Newman projection (Section 3.1) Method for depicting conformations in which one sights down a carbon-carbon bond and represents the front carbon by a point and the back carbon by a circle. 

Internal rotation in isooctane (2,2,4-trimethylpentane) creates a large number of staggered conformations. However, only rotation about the C3-C4 bond produces conformations with different structures. Plot the energy of isooctane (vertical axis) vs. HCCCtBu torsion angle, i.e., about the C3-C4 bond (horizontal axis). How many minimum energy structures are there Are they all fully staggered Draw Newman projections that show the conformation of these structures. How does steric repulsion affect isooctane conformation ... 

Conformational isomers are represented in two ways, as shown in Figure 3.6. A sawhorse representation views the carbon-carbon bond from an oblique angle and indicates spatial orientation by showing all C-Tl bonds. A Newman projection views the carbon-carbon bond directly end-on and represents the two carbon atoms by a circle. Bonds attached to the front carbon are represented by lines to the center of the circle, and bonds attached to the rear carbon are represented by lines to the edge of the circle. 

Despite what we ve just said, we actually don t observe perfectly free rotation in ethane. Experiments show that there is a small (12 kj/mol 2.9 kcal/mol) barrier to rotation and that some conformers are more stable than others. The lowest-energy, most stable conformer is the one in which all six C-H bonds are as far away from one another as possible—staggered when viewed end-on in a Newman projection. The highest-energy, least stable conformer is the one in which the six C-H bonds are as close as possible—eclipsed in a Newman projection. At any given instant, about 99% of ethane molecules have an approximately staggered conformation... 

Figure 3.8 Newman projections of propane showing staggered and eclipsed conformations. The staggered conformer is lower in energy by 14 kJ/mol.

Sighting along the C1-C2 bond of 1-clUoropropane, draw Newman projections of the most stable and least stable conformations. 

I Sight along the C2-C3 bond of 2,3-dimethylbutane, and draw a Newman projection of the most stable conformation. 

Increased substitution around a bond leads to increased strain. Take the four substituted butanes listed below, for example. For each compound, sight along the C2-C3 bond and draw Newman projections of the most stable and least stable conformations. Use the data in Table 3.5 to assign strain energy values to each conformation. Which of the eight conformations is most strained Which is least strained ... 

Figure 4.5 The conformation of cyclobutane. Part (c) is a Newman projection along the C1-C2 bond, showing that neighboring C—H bonds are not quite eclipsed.

What stereochemistry do you expect for the alkene obtained by E2 elimination of, (lR /O-LZ-dibromo-l -diphenylethane Draw a Newman projection of the reacting conformation. 

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