1,4-ditertbutylhexane conformational analysis

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cis-1,4-ditertbutylcyclohexane conformational analysis

Purpose

The purpose of this conformational analysis is to determine the preferred shape (global minimum in terms of total steric energy for the molecule) of cis-1,4-ditertbutylcyclohexane by energy minimization using Chem3D.

Introduction & Technique

PRIOR KNOWLEDGE AND PREDICTION
    cis-1,4-ditertbutylcyclohexane is a derivative of hexane, which has chair, boat, or twistboat conformers. Typically, for cyclohexane and most of its derivatives, the chair conformer is most stable (lowest energy), and the boat conformer is the least stable (highest energy). When large substituents are present on the cyclohexane ring, the molecule will prefer the chair conformer that places the substituent in the equatorial, rather than the axial position because doing so minimizes steric interactions (thereby lowering energy).
    In the case of cis-1,4-ditertbutylcyclohexane, the chair conformation of cyclohexane would place one of the tertbutyl groups axial and the other equatorial. The tertbutyl group is so large that placing it in the axial position drastically increases the molecule's energy. Therefore, the best solution is NOT a chair conformer, but rather a twist boat conformation that would allow both tertbutyl groups to be equatorial.

TECHNIQUE AND PROBLEMS
    To reach the global minimum energy, the molecule must be in a configuration so that the closest minimum is the global minimum, rather than a local minimum (i.e. one should try one's best to manipulate the molecule as close as possible to the shape that will provide the lowest energy). This was accomplished by taking the minimum energy stretched chair conformer (a local minimum) and drastically moving the ends of the hexane into a boat and moving the tertbutyl groups into equatorial positions far from each other. The 'minimize energy' command then produced the global minimum, the twist boat conformer with two equatorial tertbutyl groups.

ENERGY OF CHAIR	PICTURE OF CHAIR (Local minimum)	BEST GUESS (before minimization)
Stretch: 2.5010 Bend: 5.9431 Stretch-Bend: 0.5663 Torsion: 7.0406 Non-1,4 VDW: -1.8877 1,4 VDW: 10.4147 Total: 24.5779 Calculation completed

Result

The resulting conformer had an energy of 23.9623 kcal/ mol.

Calculation started
Note: All parameters used are finalized (Quality = 4).
Iteration    1: Minimization terminated in error because of an insignificant change in the varying measurements
Stretch:       2.3278
Bend:          3.8731
Stretch-Bend: 0.4817
Torsion:       8.4195
Non-1,4 VDW:   -2.1693
1,4 VDW:       11.0295
Total:           23.9623
Calculation completed