Ethane Newman projection

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... 

FIGURE 1.2 Newman projections for ethane, (a) Staggered (b) eclipsed. 

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. 

Figure 3.6 A sawhorse representation and a Newman projection of ethane. The sawhorse representation views the molecule from an oblique angle, while the Newman projection views the molecule end-on. Note that the molecular model of the Newman projection appears at first to have six atoms attached to a single carbon. Actually, the front carbon, with three attached green atoms, is directly in front of the rear carbon, with three attached red atoms.

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... 

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 4.5 a) The staggered conformation of ethane, b) The Newman projection formula for the staggered conformation. 

Figure 11.18 Two conformational isomers of ethane, C2H6. (a) Sawhorse representation and (b) Newman projections.

For an ethane-type molecule, a Newman projection is written and the... 

The two representations shown here are actually two different conformers of ethane there will be an infinite number of such conformers, depending upon the amount of rotation about the C-C bond. Although there is fairly free rotation about this bond, there does exist a small energy barrier to rotation of about 12kJmol due to repulsion of the electrons in the C-H bonds. By inspecting the Newman projections, it can be predicted that this repulsion will be a minimum when the C-H bonds are positioned as far away from each other... 

The many different conformers resulting from rotation around the carbon-carbon bonds in simple molecules like ethane and w-butane may be shown by Newman projections (Figure 2.7). The most stable is the anti or trans projection where the steric hindrance is minimized. There are a number of eclipsed and gauche arrangements of which only one of... 

FIGURE 2.7 (a) potential energy profile illustrating the potential energy changes associated with rotation around a C-C bond of ethane (b) Newman projections of designated conformers of n-butane. 

A Newman projection is obtained by viewing a molecule along a bond. Take the ethane (or substituted ethane) molecule represented below (a). This is seen in perspective (b). 

It should also tye noted that there is a slight preference for chain conformations of lower energy. Newman projections for the two extreme conformations of ethane are shown in Figure 1.2. The value of 2.8 kcal/mol represents the energy opposing free rotation about the C—C bond. 

Visualization of conformers There are four conventional methods for visualization of three-dimensional structures on paper. These are the ball and stick method, the sawhorse method, the wedge and broken line method and the Newman projection method. Using these methods, the staggered and eclipsed conformers of ethane can be drawn as follows. 

Draw, using dash-wedge, sawhorse, or Newman projection formulas, the important conformations of ethane, propane, butane, and various halogenated derivatives of these alkanes. 

Fig.I. Newman projections of the staggered (I) and eclipsed (II) conformations of ethane.

Each carbon-hydrogen bond on one carbon of the eclipsed conformation of ethane is directly in line with a carbon-hydrogen bond on the other carbon. This is easier to see if we view the molecule end-on—that is. down the carbon-carbon bond. A drawing of such a view, called a Newman projection, is as follows ... 

In the staggered conformation of ethane the carbon-hydrogen bond on one carbon bisects the angle between two of the carbon-hydrogen bonds on the other carbon. Again, this can better be seen in the Newman projection ... 

Eclipsed and staggered conformations of ethane and their Newman projections. 

Conformational studies on ethane-1,2-diol (HOCH2—CH2OH) have shown the most stable conformation about the central C—C bond to be the gauche conformation, which is 9.6 kJ/mol (2.3 kcal/mol) more stable than the anti conformation. Draw Newman projections of these conformers and explain this curious result. 

Newman projections for the staggered and eclipsed conformations of ethane are shown below. 

Figure 4.6 illustrates the Newman projections for both the staggered and eclipsed conformations for ethane. 

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