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Alkanes can have different conformations. By analyzing the structure of ethane, we can define certain aspects regarding its conformations. Conformations are different arrangements of the atoms in a molecule, as a result of rotation around a single bond.

image\25509 ch 15.png

In staggered conformation, the torsional angle is 600. In eclipsed conformation, each carbon-hydrogen bond is aligned with the carbon-hydrogen bond of the next carbon.

image\25518 ch 15.png

In eclipsed conformations, the torsional angle is 0o. In staggered conformations, the torsional angles can either be 60o (gauche) or 180o (anti). The anti conformation is more stable than the gauche conformation. We should also consider the fact that in this analysis of staggered conformation, the ethane molecule looks the same in the Newman projections, whether it is gauche or anti. Reason: There are no substituents other than just hydrogens. To denote the positional significance, the hydrogens are indicated in bold in the diagrams shown in Figure 15-4.


image\25527 ch 15.png

Figure 15-3

image\25536 ch 15.png

Figure 15-4

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