Conformation, molecular Newman projection

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... 

Newman projections, looking along the central C2—C3 bond, for four conformations of butane. Construct butane with your molecular models, and sight down the C2—C3 bond. Notice that we have defined the dihedral angle d as the angle between the two end methyl groups. 

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. 

FIGURE 12.1. Ways of depicting molecular conformation, (a) Perspective view and (b) Newman projection. 

The conformation of cyclobutane. Part (a) shows computer-generated molecular models. Part (c) is a Newman projection along the C1-C2 bond, showing that neighboring C-H bonds are not quite eclipsed. 

The boat conformation of cyclohexane (18) can be constructed from a molecular model of the chair form by holding the right-hand three carbons C(2), C(3) and C(4) of 15, clamped from the top with the hand and moving the left-hand three carbons upward. A Newman projection of the boat form looking along the C(l)-C(2) bond, and shown in 19, is reminiscent of the highest energy cis conformation of butane. 

Let us now consider which bonding model is more amenable to qualitative predictions of molecular conformation. Specifically, what should be the preferred conformation of propene Walters noted that two conformers of propene (designated as I and II) can be visualized as Newman projections observed by sighting down the C3-C2 bond (Figiue 1.33). In conformer I, a C-H bond eclipses a carbon-carbon double bond, hi conformer II, a C-H bond eclipses a C—H bond. Assuming that there is greater electron density in a double bond than in a C-H single bond, we would expect conformer II to be more stable. Experimentally, however, conformer I was found to be more stable by about 2 kcal/mol. 

Which of the following two conformations is the more stable Hint Use molecular models or draw Newman projections looking down the bond being... 

Molecular models and Newman projections of the staggered and eclipsed conformations of ethane. The dihedral angle in the staggered form is 60T and that in the eclipsed form is 0°. The C—C bond is rotated slightly in the Newman projection of the eclipsed form in order to show the H atoms attached to the back C atom. 

The diastereotopic nature of H and H at C3 in 2-butanol can also be appreciated by viewing Newman projections. In the conformations shown below (Fig. 9.16), as is the case for every possible conformation of 2-butanol, H and H experience different environments because of the asymmetry from the chirality center at C2. That is, the molecular landscape of 2-butanol appears different to each of these diastereotopic hydrogens. H and H experience different magnetic environments, and are therefore not chemical shift equivalent. This is true in general diastereotopic hydrogens are not chemical shift equivalent. 

Figure 9.16 and H (on C3, the front carbon in the Newman projection) experience different environments in these three conformations, as well as in every other possible conformation of 2-butanol, because of the chirality center at C2 (the back carbon in the Newman projection). In other words, the molecular landscape as viewed from one diastereotopic hydrogen will always appear different from that viewed by the other. Hence, and experience different magnetic environments and therefore should have different chemical shifts (though the difference may t>e small). They are not chemical shift equivalent. 

The molecular model of S20 F2, the characterization of the angles of rotation, and the Newman projections of the conformer established by Hencher and Bauer (1973)... 

The molecular models and Newman projections of two conformers of fluorosulphuric acid methyl ester and chlorosulphuric acid methyl ester... 

FIGURE 14.5 Extended Newman projection along the long molecular axes of 1 and 2 showing main conformations. The bars represent the substituent and the circle is the nitrogen or sulfur atom. 

Alkanes are compounds in which stereochemical differentiation results from their various possible conformations. A number of these compounds, especially with different substituents present in the chain, may exist as enantiomers or diastereoi-somers, that is, compounds that are optically active. Figure 2.10 shows Newman projections of six different conformations for a molecular fragment consisting of two adjacent carbon atoms with substituents. 

One conformation appears to be lowest in energy. This conformation is shown as molecular model 48D in the same Newman-type projection as 48C. This conformation is thought to look a little like an easy chair, and it is called a chair conformation. The chair shape can be seen in 48B or in molecrdar model 48E, which views the conformation from the side. Note that 48D is the molecular model of48B, and that 48D and 48E are identical, but simply viewed from a different perspective. 

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