Use the ChemDraw and Chem3D software for the following questions:
1. Bredt's rule states that elimination in a bridged bicyclic system gives a double bond away from the bridgehead position. This preference diminishes with ring size. Use molecular mechanics with the MM2 force field to minimize the energy of structures 2 and 3 for bridge sizes ranging from n=1 to n=4. Report the steric energies of both structures as well as the differences (make sure that you have obtained the lowest energy structure in each case); examine how the dihedral angle of the double bonds changes with bridge size. Discuss how this relates to Bredt's rule.
Calculated energy difference (3 - 2, kcal/mol)
| Theory | n=1 | n=2 | n=3 | n=4 |
| MM2 Struc 2 | ||||
| MM2 Struc 3 | ||||
| MM2 diff | ||||
| AM1 diff | 53.84 | 53.27 | 27.01 | 11.51 |
| PM3 diff | 50.79 | 49.10 | 24.55 | 9.42 |
| MNDO/3 diff | 40.29 | 46.37 | 28.37 | 16.27 |
| HF/3-21G diff | 66.0 | 66.6 | 31.2 | 13.2 |
2. Calculate the difference in steric energy for cis and trans cycloalkenes with n=6 (cyclohexene) to n=9 (cyclononene). Report the steric energies of both structures as well as the differences (make sure that you have obtained the lowest energy structure in each case); examine how the dihedral angle of the double bonds changes with ring size. Discuss how this relates to the stability of trans cycloalkenes.
Calculated energy difference (trans - cis, kcal/mol)
| Theory | n=6 | n=7 | n=8 | n=9 |
| MM2 cis | ||||
| MM2 trans | ||||
| MM2 diff | ||||
| AM1 diff | 59.36 | 30.90 | 14.05 | 4.96 |