To visualize the chair conformations of cis-1,2-dimethylcyclohexane and trans-1,2-dimethylcyclohexane, let’s first outline what each conformation looks like:
Cis-1,2-Dimethylcyclohexane
In the cis isomer of 1,2-dimethylcyclohexane:
- Both methyl groups are present on the same side of the ring.
Chair Conformation 1:
- Both methyl groups are in equatorial positions. This is the most stable conformation.
- In this conformation, the steric hindrance is minimized as the bulky groups are positioned away from the ring.
Chair Conformation 2:
- One methyl group is equatorial, and the other is axial. This conformation is less stable due to increased steric strain, as the axial methyl experiences 1,3-diaxial interactions.
Trans-1,2-Dimethylcyclohexane
In the trans isomer of 1,2-dimethylcyclohexane:
- The two methyl groups are on opposite sides of the ring.
Chair Conformation 1:
- One methyl group is in an equatorial position, and the other is in an axial position. This is less stable due to steric hindrance but is still a possible conformation.
Chair Conformation 2:
- Both methyl groups are in equatorial positions. This is the most stable conformation since it minimizes steric strain.
Comparison of Stability
- Cis-1,2-dimethylcyclohexane: The most stable conformation has both methyl groups in equatorial positions. The conformer with one axial and one equatorial methyl group is less stable due to steric strain.
- Trans-1,2-dimethylcyclohexane: The most stable conformation also has both methyl groups in equatorial positions, which minimizes steric strain. The conformation with one methyl in the axial position is less stable due to 1,3-diaxial interactions.
Summary:
While both cis and trans dimethylcyclohexanes can adopt conformations with both methyl groups in equatorial positions (which are the most stable), the cis isomer experiences greater steric strain in one of its conformations when compared to the trans isomer. Thus, the cis isomer is generally less stable compared to the trans isomer due to the presence of steric hindrance in its axial position. The most stable conformation for both is when both methyl groups are equatorial.