Ethane, bond angles eclipsed conformation

Conformational isomerism can be presented with the simplest example, ethane (C2Hg), which can exist as an infinite number of conformers by the rotation of the C-C ct bond. Ethane has two sp -hybridized carbon atoms, and the tetrahedral angle about each is 109.5°. The most significant conformers of ethane are the staggered and eclipsed conformers. The staggered conformation is the most stable as it has the lowest energy. 

Rotating the atoms on one carbon by 60° converts an eclipsed conformation into a staggered conformation, and vice versa. These conformations are often viewed end-on— that is, looking directly down the carbon-carbon bond. The angle that separates a bond on one atom from a bond on an adjacent atom is called a dihedral angle. For ethane in the staggered conformation, the dihedral angle for the C-H bonds is 60°. For eclipsed ethane, it is 0°. 

Ethane is the simplest hydrocarbon that can have distinct conformations. Two, the staggered conformation and the eclipsed conformation, deserve special attention and are illustrated in Figure 3.1. The C—H bonds in the staggered conformation are arranged so that each one bisects the angle made by a pair of C—H bonds on the adjacent carbon. In the eclipsed conformation each C—H bond is ahgned with a C—H bond on the adjacent carbon. The staggered and eclipsed conformations interconvert by rotation around the carbon-carbon bond. Different conformations of the same molecule are sometimes called conformers or rotamers. 

Of the two conformations of ethane, the staggered is more stable than the eclipsed. The measured difference in potential energy between them is 12 kJ/mol (2.9 kcal/mol). A simple explanation has echoes of VSEPR (Section 1.10). The staggered conformation allows the electron pairs in the C—H bonds of one carbon to be farther away from the electron pairs in the C—H bonds of the other than the eclipsed conformation allows. Electron-pair repulsions on adjacent carbons govern the relative stability of staggered and eclipsed conformations in much the same way that electron-pair repulsions influence the bond angles at a central atom. 

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