Staggered conformations of ethane

FIGURE 3 2 Some commonly used draw mgs of the staggered conformation of ethane... 

Staggered conformation of ethane (most stable conformation)... 

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

Bickelhaupt, F. M. Baerends, E. J. The case for steric repulsion causing the staggered conformation of ethane. Angew. Chem. Int. Ed. 

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. 

There are two more important reasons why the staggered conformation of ethane is lower in energy than the eclipsed conformation. The first is that the electrons in the bonds repel each other and this repulsion is at a maximum in the eclipsed conformation. The second is that there may be some stabilizing interaction between the C-H G bonding orbital on one carbon and the C-H C antibonding orbital on the other carbon, which is greatest when the two orbitals are exactly parallel this only happens in the staggered conformation. 

Fig. 6,11. Energy changes in kJ/mol for the rotation of the staggered conformer of ethane (S) into the eclipsed conformer (E) and into the frozen eclipsed conformer (FE).

The instability of the eclipsed form of ethane was originally postulated to result from repulsion of filled hydrogen orbitals. However, state-of-the-art quantum chemical calculations now indicate that two main factors contribute to the preference for the staggered conformation of ethane. First, the eclipsed form is selectively destabilized by unfavorable four-electron interactions between the filled C-H bonding orbitals of... 

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