Totally eclipsed conformations

The totally eclipsed conformation is about 6 kJ (1.4 kcal) higher in energy than the other eclipsed conformations because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion. This kind of interference... 

Torsional energy of butane. The anti conformation is lowest in energy, and the totally eclipsed conformation is highest in energy. 

Rotating the totally eclipsed conformation 60° to a gauche conformation releases most, but not all, of this steric strain. The gauche conformation is still 3.8 kJ (0.9 kcal) higher in energy than the most stable anti conformation. 

For more than one asymmetric carbon atom, the Fischer projection represents a totally eclipsed conformation.This is not the most stable conformation, but it s usually the most symmetric conformation, which is most helpful for comparing stereochemistry. 

Torsional strain is the second factor in cyclopropane s large ring strain. The three-membered ring is planar, and all the bonds are eclipsed. A Newman projection of one of the carbon-carbon bonds (Fig. 3-16) shows that the conformation resembles the totally eclipsed conformation of butane. The torsional strain in cyclopropane is not as great as its angle strain, but it helps to account for the large total ring strain. 

Organic compounds commonly exist as rapidly interconverting chiral conformations. Even ethane is chiral in its skew conformations. When we speak of chirality, however, we intend to focus on observable, persistent properties rather than transient conformations. For example, butane exists in gauche conformations that are chiral, but they quickly interconvert. They are in equilibrium with the totally eclipsed conformation, which is symmetric, implying that butane must be achiral. 

FIGURE 26-9 Steric hindrance in a totally eclipsed conformation of butane... 

In the totally eclipsed conformation, the two methyl groups interfere with each other. 

When these molecules buckle, some or all of the hydrogen atoms move slightly out of the totally eclipsed conformation, relieving repulsion associated with the eclipsing of carbon-hydrogen bonds but at the expense of decreasing the already strained C—C—C bond angles even further. 

---