Conformations of ethane

The origin of a torsional barrier can be studied best in simple cases like ethane. Here, rotation about the central carbon-carbon bond results in three staggered and three eclipsed stationary points on the potential energy surface, at least when symmetry considerations are not taken into account. Quantum mechanically, the barrier of rotation is explained by anti-bonding interactions between the hydrogens attached to different carbon atoms. These interactions are small when the conformation of ethane is staggered, and reach a maximum value when the molecule approaches an eclipsed geometry. 

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

Of the two conformations of ethane the staggered is 12 kJImol (2 9 heal mol) more stable than the eclipsed The staggered conformation is the most stable conformation the eclipsed is the least stable conformation Two main explanations have been offered for the difference in stability between the two conformations One explanation holds that repulsions between bonds on adjacent atoms destabilize the eclipsed conformation The other suggests that better electron delocalization stabilizes the staggered conformation The latter of these two explanations is now believed to be the correct one... 

Section 3 1 The most stable conformation of ethane is the staggered conformation It IS approximately 12 kJ/mol (3 kcal/mol) more stable than the eclipsed, which IS the least stable conformation... 

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

Torsional strain refers to the component of total molecular energy that results from nonoptimal arrangement of vicinal bonds, as in the eclipsed conformation of ethane. The origin and stereoelectronic nature of torsional strain were discussed in Section 1.1.1. The... 

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 3.1 The staggered and eclipsed conformations of ethane shown as ball-and-spoke models (left) and as space-filling models (right). 

FIGURE 3.3 Some commonly used drawings of the eclipsed conformation of ethane. 

Section 3.1 The most stable conformation of ethane is the staggered conformation. 

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

Still another way to picture the conformational preferences is to visualize the C=C double bond in terms of two equivalent banana bonds (Fig. 3.50). In this picture the preferred conformations are those with C—F in staggered orientation with respect to the three bonds (two banana bonds and one C—H bond) of the vinyl moiety, analogous to the preferred conformations of ethane. However, in using this ethane-like mnemonic one should recall that its essential origin lies in the hyperconjugative interactions of E(SL> rather than the steric and electrostatic interactions of (L). 

Figure 3.55 Leading cjch-och hyperconjugative donor-acceptor interactions in the staggered (left) and eclipsed (right) conformers of ethane.

Figure 4.81 Staggered and eclipsed conformers of ethane-like Os2H6.

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