Torsional strain in ethane

Conformations in which the torsion angles between adjacent bonds are other than 60° are said to have torsional strain. Eclipsed bonds produce the most torsional strain staggered bonds none. Because three pairs of eclipsed bonds are responsible for 12 kJ/rnol (2.9 kcal/rnol) of torsional strain in ethane, it is reasonable to assign an energy cost of 4 kJ/rnol (1 kcal/rnol) to each pair. In this chapter we ll learn of additional sources of strain in molecules, which together with torsional strain comprise steric strain. 

The origin of the torsional strain in ethane remains controversial. As rotation to the eclipsed geometry brings pairs of C-H bonds on the two carbons closer to each other, repulsion between the electrons in these bonds increases. Rotation also causes subtle changes in molecular orbital interactions, weakening the C-C bond in the eclipsed conformation. The relative importance of these effects has been a matter of debate for decades, with the most recent published theoretical research (2007) favoring electron repulsion as the major contributor to the rotational energy. 

The difference in energy between the staggered and eclipsed forms is due almost entirely to the torsional strain in the eclipsed conformation. At any instant, almost all the molecules of ethane reside in the staggered 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... 

Each H <— H eclipsing interaction in ethane costs about 4.0 kj/mol. Wow many such interactions are present in cyclopropane What fraction of the overall 115 kj/mol (27.5 kcal/mol) strain energy of cyclopropane is due to torsional strain ... 

Torsional energy and torsional strain Torsional energy is the energy required for rotating about the C-C a bond. In ethane, this is very low (only 3 kcal). Torsional strain is the strain observed when a conformer rotates away from the most stable conformation (i.e. the staggered form). Torsional strain is due to the slight repulsion between electron clouds in the C-H bonds as they pass close by each other in the echpsed conformer. In ethane, this is also low. 

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