Butane Conformation Case Study

When butane’s dihedral angle was at 0⁰ and 360, the conformations are at an Emax. This means that the two methyl groups are eclipsed. At 60⁰ and 300⁰, which have gauche conformations, the methyl groups are more shifted and the potential energy difference is 18.2 kj/mol. At 120⁰ and 240⁰, eclipsed again, the energy rises to -4.6788 kj/mol, from its previous -17.721 kj/mol, the hydrogens can be found lying on the same plane, increasing the energy. Yet, the energy is not as high as it is at 0⁰ and 360⁰. For it to reach its Emax, at 0.5593 kj/mol, energy will have to increase by 5.23 kj/mol. At 180⁰, there is an Emin, which demonstrates the methyl groups being furthest apart from each other. This is the most stable conformation of butane.
Similarly
for 1,2-dicholoroethane, at 0⁰ and 360⁰, the two chlorine substituents are eclipsing one another, giving the highest Emax. At 0⁰ and 360⁰, the chlorine substituents are also cis to one another. At 60⁰ and 300⁰, the chlorine substituents are more trans to one another. The energy here is lower, by ∆33.78kj/mol, but it is not yet at its Emin. At 120⁰ and 240⁰, the chlorine substituents are in the same plane. At 120⁰ and 240⁰, the chlorines eclipse once more increasing the energy by ∆15.06kj/mol. Lastly, at 180⁰ an Emin is observed, because there is an anti-conformer. The chlorine substituents are farthest away from each other and are in the same plane. The energy therefore has decreased by 21.65kj/mol.
All the eclipsed conformers are going to have the same energy, and likewise, the staggered. There is free rotation about the sigma bonds that form between the carbons. This rotation also gives way to the geometric isomers that form. Since the energies were lowest at 180⁰, the anti-conformations are found to be the most stable. The eclipsed conformations for butane and 1,2-dicholoroethane had Emax because the carbon sigma bonds are overlapping, causing repulsive interactions between the electrons. This makes this conformation less energetically favorable and less stable. At these two energy minima, the anti and gauche forms, are in staggered positions and have no torsional strain (arises from eclipsed bonds) therefore. However, there is a bit of steric strain in the gauche form. So for that case the anti-form will have the lowest energy minimum.
Names of assigned substituted cyclohexanes
R group=CH3
A: trans-1-tert-butyl-4-methylcyclohexane
B: cis-1-tert-butyl-4-methylcyclohexane
C: trans-1-tert-butyl-2,4,6-trimethylcyclohexane
D: cis-1-tert-butyl-2,4,6-trimethylcyclohexane
E: trans-1-tert-butyl-3,4,5-trimethylcyclohexane
F: cis-1-tert-butyl-3,4,5-trimethylcyclohexane

The conformation of structure A required the least energy overall.
Structure D’s conformation required the greatest energy overall. Structures A and B, structures C and D, and structures E and F are diastereomers of each other. The cis structure of each pair seems to require greater energy for all pairs. The energy differences between structures A and B is about ∆6kj/mol, structures C and D have energy differences of about ∆20kj/mol and structures E and F, ∆15 kj/mol. Structure A, compared to B, had less energy because the R group is in equatorial, the same for structures E, in the among the E and F
diastereomers.
Among structures C,D, E and F, D’s conformation required the greatest energy and E had the least.
Since structure D had all substituents in axial position, this position is least stable. Structural E, had all substituents in equatorial position, which is more energetically favorable because there is less steric strain. Steric strain is when there is repulsion between the electron clouds of the groups or atoms. This arises, especially in structure D, when the hydrogens attached in the axial methyl substituents interact with one another. For cyclohexanes it is best when any of the substituents lie in equatorial position as much as possible, because it gives way to less steric strain. When the cyclohexanes are poly-substituted, not all the isomers can occupy all equatorial positions, yet the most stable isomer will have substituents in equatorial positions. The trans isomers, compared to the cis isomers have the greatest energy. Structures C and F’s cis isomers are less thermodynamically stable than their trans. The cis and trans isomer are not the same isomer in different structural conformations, and nor are they able to readily convert by rotating them. The bonds will have to be broken for this to
occur.