Although steric and/or bond electron repulsion remain the most popular explanation for the hindered rotation of ethane, molecular orbital interactions have also been proposed as a significant factor. For a discussion of this feature .
Potential Energy Profile for Ethane Conformers
The above animation illustrates the relationship between ethane's potential energy and its dihedral angle
The hydrocarbon butane has a larger and more complex set of conformations associated with its constitution than does ethane. Of particular interest and importance are the conformations produced by rotation about the central carbon-carbon bond. Among these we shall focus on two staggered conformers (A & C) and two eclipsed conformers (B & D), shown below in several stereo-representations. As in the case of ethane, the staggered conformers are more stable than the eclipsed conformers by 2.8 to 4.5 kcal/mol. Since the staggered conformers represent the chief components of a butane sample they have been given the identifying prefix designations anti for A and gauche for C.
|Four Conformers of Butane|
The following diagram illustrates the change in potential energy that occurs with rotation about the C2–C3 bond. The model on the right is shown in conformation D, and by clicking on any of the colored data points on the potential energy curve, it will change to the conformer corresponding to that point. The full rotation will be displayed by turning the animation on. This model may be manipulated by click-dragging the mouse for viewing from any perspective.
|Potential Energy Profile for Butane Conformers|
Stick Model Animation on/off
(i) Most conformational interconversions in simple molecules occur rapidly at room temperature. Consequently, isolation of pure conformers is usually not possible.
(ii) Specific conformers require special nomenclature terms such as staggered, eclipsed, gauche and anti when they are designated.
(iii) Specific conformers may also be designated by dihedral angles. In the butane conformers shown above, the dihedral angles formed by the two methyl groups about the central double bond are: A 180º, B 120º, C 60º & D 0º.
(iv) Staggered conformations about carbon-carbon single bonds are more stable (have a lower potential energy) than the corresponding eclipsed conformations. The higher energy of eclipsed bonds is known as eclipsing strain.
(v) In butane the gauche-conformer is less stable than the anti-conformer by about 0.9 kcal/mol. This is due to a crowding of the two methyl groups in the gauche structure, and is called steric strain or steric hindrance.
(vi) Butane conformers B and C have non-identical mirror image structures in which the clockwise dihedral angles are 300º & 240º respectively. These pairs are energetically the same, and have not been distinguished in the potential energy diagram shown here.
For a more extensive discussion of rotamer analysis Click Here.
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