Sawhorse representation ethane

The formalism that we have set up to describe chain flexibility readily lends itself to the problem of hindered rotation. Figure 1.8a shows a sawhorse representation of an ethane molecule in which the angle of rotation around the bond is designated by electron repulsion between the atoms bonded to... 

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.

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

Let us now consider rotation about the central C-C bond in butane. Rotation about either of the two other C-C bonds will generate similar results as with ethane above. Wedge-dot, Newman, and sawhorse representations are all shown use the version that appears most logical to you. 

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. 

Ethane can exist in an infinite number of conformations. The conformation in which the hydrogen atoms and the bonding electrons are the farthest away firom one another has the lowest energy. This conformation is sta ered. The conformation in which the hydrogen atoms are closest to one another has the highest energy. This conformation is eclipsed. In the echpsed conformation, each C—H bond on one carbon atom lines up with a C—H bond on another carbon atom, as the moon sometimes eclipses the sun. Sawhorse representations of the conformations of ethane are shown in Figure 4.2. These representations are three-dimensional and show the carbon—carbon bond as well as all of the C—H bonds. 

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 4.6. Sawhorse and Newman representations for staggered and eclipsed conformers of ethane (CH3CH3).

Fischer projections do not attempt to represent energy minimum conformations. Instead, they are designed to help distinguish the stereoisomers. An analogy would be the representation of substituted ethanes in an eclipsed conformation, called the sawhorse stmcture (p. 65). Fischer projections are used because they are simple to draw and read. We will see soon how to translate them into more accurate representations of molecules. 

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