Conformational Analysis of Ethane, Butane and 2-Butanol

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This Demonstration shows the dependence on the C-C bond torsion angle of the potential energy of three open-chain organic compounds, which results from steric effects among the various atoms or groups within the molecule.

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Three representative molecules exhibiting these effects are considered: ethane, n-butane and 2-butanol. You can rotate one part of the molecule relative to the other and follow the rotation both in the 3D representation and the Newman projection along the C-C axis.

You can see the effect of the rotation on the potential energy by following the moving point on the plot; the abscissa shows the rotation angle in degrees, while the ordinate shows the potential energy as a percentage [1].

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Contributed by: D. Meliga, S. Z. Lavagnino and A. Ratti (November 2016)
Additional contribution by: G. Follo
Open content licensed under CC BY-NC-SA


Snapshots


Details

Three possibilities are analyzed: periodic potential generated by a molecule where just hydrogen atoms (white spheres) are bonded to the carbon atoms (black spheres), symmetric potential generated by the substitutions of two hydrogen atoms with two groups (yellow spheres), and asymmetric potential generated by further substituting an group (red spheres).

Snapshot 1: ethane molecule with a torsion angle of 60° (staggered) relative to the initial conformation (torsion angle 0°, eclipsed) representing the absolute minimum of potential energy

Snapshot 2: n-butane molecule with a torsion angle of 180° (anti) representing the absolute minimum of potential energy

Snapshot 3: 2-butanol molecule with a torsion angle of 60° (gauche) representing a relative minimum of potential energy, different from the 300° relative minimum because of the group

Reference

[1] G. Natta and M. Farina, Stereochemistry, New York: Harper and Row, 1972.



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