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Alkanes: Introduction and Chemical Bond

Alkanes: Conformation

Stereochemistry in itself, i.e. the three-dimensional structure, does not provide a complete description of the properties of alkanes. In addition, structural dynamics and flexibility have to be considered. Even the small molecule ethane is not a rigid molecule. The rotation of the two CH3 groups around the central C-C single bond of ethane has a very low energy barrier. In particular, the sawhorse projection and the Newman projection are especially suited to visualize the conformational flexibility.

Conformations of ethane

The staggered conformation with a dihedral angle of 60° is preferred over the eclipsed one in which the hydrogens at the forward carbon are directly in front of those on the back carbon. This is due mostly to the long-distance repulsion of the H atoms. The energy barrier of the rotation around the central C-C single bond is only 12 kJmol-1 . This results in fast rotation of the CH3 groups even at very low temperatures. Plotting the dihedral angle of an arbitrary H-C-C-H unit of ethane against the energy yields a sinus-shaped conformational energy profile. The staggered conformations at 60°, 180° and 300° are not distinguishable due to the symmetry of the molecule. The eclipsed conformations are situated as energy maxima between these minima. Such maxima are called transition states in one-dimensional energy profiles.

Energy of ethane as a function of rotation around the central C-C bond
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