Newman projections of ethane

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.

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. 

The structural feature that Figures 3.2 and 3.3 illustrate is the spatial relationship between bonds on adjacent carbons. Each H—C—C—H unit in ethane is characterized by a torsion angle or dihedral angle, which is the angle between the H—C—C plane and the C—C—H plane. The torsion angle is easily seen in a Newman projection of ethane as the angle between C—H bonds of adjacent carbons. 

A Newman projection of ethane, showing the front carbon and the back carbon. 

The dihedral angle between two hydrogen atoms in a Newman projection of ethane. 

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. 

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

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. 

Consideration of the Newman projection of propane (9) suggests that the energy profile associated with rotation around the CH3-CH2 bond contains both three identical maxima and three identical minima per revolution. In eclipsed conformations there is now a non-bonded repulsion between a methyl and a hydrogen accordingly, the torsional barrier, at 14.2 kJ mol1, is a little higher than in ethane. 

SAMPLE SOLUTION (a) The Newman projection of this alkane resembles that of ethane except one of the hydrogens has been replaced by a methyl group. The drawing is a Newman projection of propane, CH3CH2CH3. 

Fig. 9.1. Newman projections of various conformations of 1,1,2,2-tetraalkylated ethane qualitative terms for the description of the conformations are ...

Fig. 9.2. Newman projections of major calculated conformations (X-ray data for 5). a 1,1,2,2-tetramethyl ethane and disilane relation sc to p = 2 1 b 1,1,2,2-tetra-rert-butyl ethane and disilane c 1,1,2,2-tetra-cyclohexyl ethane, disilane and diphosphine...

Figure 3.5 shows a ball-and-stick model and a Newman projection of an eclipsed conformation of ethane. In this conformation, the three C—H bonds on one carbon are as close as possible to the three C—H bonds on the adjacent carbon. In other words, hydrogen atoms on the back carbon are eclipsed by the hydrogen atoms on the front carbon. 

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. 

A Newman projection of the echpsed conformation of ethane shows only the three C—H bonds of the ftont carbon atom. The bonds and hydrogen atoms at the back are hidden by the front echpsmg bonds and hydrogen atoms. However, the bonded hydrogen atoms of the back carbon atom can be shown by viewing the conformation shghtly off the bond axis so that all bonds can be seen. 

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... 

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. 

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. 

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