Butane, sawhorse projection

Other common methods for representing the three-dimensional structures of molecules include Newman projections for showing conformational relationships and sawhorse figures. Newman projections look down a carbon-carbon bond so that the front carbon, designated by a circle, obscures the carbon directly behind it. Valences (bonds) to the front carbon extend to the center of the circle, while bonds to the rear carbon stop at the circle. Sawhorse projections have the carbon-carbon bond at oblique angles, which attempts to represent a perspective drawing of the molecule. Thus for 2-chloro butane, if one chooses to examine the 2,3 bond, then the sawhorse and Newman projections would be... 

Considerations of minimum overlap of radii of nonbonded substituents on the polymer chain are useful in understanding the preferred conformations of macromolecules in crystallites. The simplest example for our purposes is the polyethylene (1-3) chain in which the energy barriers to rotation can be expected to be similar to those in /i-butane. Figure 4-2 shows sawhorse projections of the conformational isomers of two adjacent carbon atoms in the polyethylene chain and the corresponding rotational energy barriers (not to scale). The angle of rotation is that between the polymer chain substitutents and is taken here to be zero when the two chain segments are as far as possible from each other. 

Figure 3.1. Different representations of butane (C4H10) (a) in-line type set (b) in-line condensed (c) expanded horizontally (d) expanded vertically (e) expanded three-dimensionaUy, approximately showing each carbon as a tetrahedron (f) a sawhorse projection looking along the carbon-carbon o bond connecting the second and third carbon atoms and (g) a Newman (Melvin S. Newman [1908-1993], professor,The Ohio State University) projection in which the a bond between the second and third carbon atoms is represented as a circle.

Figure 2.26 The four distinct conformations of n-butane shown as sawhorse projections (top) and Newman projections (bottom). In a Newman projection, one is looking down a C-C bond with the back carbon represented by the circle and with bonds originating in the center of the circle being attached to the closer carbon. It is a common practice to skew the groups attached to the back carbon slightly in eclipsed conformations to make the attached groups easier to see.

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

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