For the 1,3-cyclohexanes, the most stable conformation of the cis isomer has both methyl groups in equatorial positions. As you know, the geometry of sp³ hybridized carbon is tetrahedral. With cyclohexane, the whole molecule is not as flat as I often sketch in my notebook. Therefore, cyclohexane adopts two conformations. Boat form and chair form. The boat structure is less frequently seen due to its slightly greater steric hindrance, while the chair structure is preferred as it is very stable.
The figure below shows the chair conformation of cis-1 3-dimethylcyclohexane. Also, cis-1,3-dimethylcyclohexane exists in two chair forms. This is because the conformation continuously changes shape and is also called a compound in a dynamic state. The conformation is also temperature dependent. Therefore, cyclohexane shows different peaks in H NMR at low temperature and room temperature.
A closer look at cyclohexane reveals that the 12 substituents are not the same. However, six substituents occupy axial positions and the remaining six occupy equatorial positions.
The figure below shows the dynamic equilibrium conformation of cis-1 3-dimethylcyclohexane.
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