Purpose:

The purpose of this exercise is to determine the minimum energy conformation of cis-1,4-di-tert-butylcyclohexane using Chem3D.

Analysis:

The cis-1,4-di-tert-butlcyclohexane was drawn in ChemDraw and the imagine was then transferred into Chem3D. When the structure was placed into Chem3D the program automatically arranges the structure into a low energy conformation with a 1-tert-butyl group axial and one equatorial. The steric energy of the original structure Figure 1 (below) was then calculated by selecting the compute properties button. The total steric energy calculated was 530.48 kcal/mol. This is an extremely high value which means the arrangement is not in the lowest minimum energy.

Figure 1:

Original Chair Conformation

To determine a lower energy value the energy can be minimized by selecting the MM2 button which will determine the local minimum energy. Figure 2 (below) which was now rearranged in the lowest minimum energy. The molecule shifted from a chair to a stretched-chair by shifting two t-butyl groups so they are as far away from each other and the minimum steric energy was determined to be 28.35 kcal/mol. The stretched-chair now has a lower minimium energy due the the t-butyl groups now in the equatorial position which will cause less steric hendrance which means a lower energy.

Figure 2:


  Stretched-Chair Conformation

Figures 1 and 2 were the energies determined for the chair conformation. If you adjust the t-butyl group on the 4^th carbon by dragging the carbon upwards you can now transform the molecule into a boat conformation as seen in Figure 3 (below). Now that the molecule is in the boat conformation the energy can be determined for a boat conformation. The calculated energy for the boat conformation is 54.68 kcal/mole.

Figure 3:

Boat Conformation

The energy determined was not the lowest energy of the boat conformation. To determine a lower energy value the energy can be minimized by selecting the MM2 button which will determine the local minimum energy of the boat conformation. Figure 4 (below) was now arranged in the lowest minimum energy. The determination of the minimum energy changed the structure into a twist boat with a calculated minimum energy of 27.67 kcal/mole. After calculating the energies of four different structures the twisted boat conformation had a slightly lower minimum energy then the stretched chair. The slight difference in energy can be accounted for because the two different chair conformations have a t-butyl in the axial position versus the twisted boat conformation which has both t-butyl groups are in an equatorial position which would cause less steric hendrance and giving the twisted boat the lowest energy value.     

Figure 4:


Twist Boat Conformation

Conclusion:

It is usually expected that in cyclohexane ring the chair conformation would have the lowest energy of all the conformations. In this case, the twisted boat had a slightly lower conformational energy then the stretched-chair. The two t-butyl groups on the ring have at least on of those groups in axial position which would cause more steric hindrance in the chair conformation. In the twisted boat conformation both of the t-butyl groups are in a form of the equatorial position which would cause less steric hindrance which would lead to a minimum lower energy then the chair conformation.