For cis-1,3-dimethylcyclohexane, which two chair conformations are in equilibrium? A B C D E F

equilibrium

#1

For cis-1,3-dimethylcyclohexane, which two chair conformations are in equilibrium? A B C D E F

image

Concept and reason
Chirality center: An atom (usually carbon) which is bonded to four different atoms or groups is known as chiral center.

In the chair conformer, if the atoms are facing towards up, then those atoms must be placed at above the plane in cyclohexane ring structure. Similarly, if the atoms are facing towards down, then those atoms must be placed at below the plane in the cyclohexane ring structure.

Cis isomer contains two groups/substituents on the same side of the ring (either both up or both down). Trans isomer contains two groups/substituents on the opposite side of the ring (one up and one down).

Fundamentals
Cyclohexane is more stable in its chair conformation. In chair conformation of cyclohexane, two types of groups are present at each carbon atom. They are axial and equatorial groups. The six axial bonds one on each carbon, are parallel and alternate up-down.
image
The six equatorial bonds one on each carbon, come in three sets of two parallel ring bonds. Equatorial bonds alternate between sides around the ring.
image
In chair conformation, there is steric interaction developed between the axial groups at first and third positions to each other. This type of interaction is called 1,3-diaxial interaction.

Answer:
The structures of cis-1,3-dimethylcyclohexane are as follows:

image

Explanation:
The name cis represents that the both the methyl substituents must be placed either up or down on 1st and 3rd carbon atoms.

The ring flipping is as follows:

Explanation:
At room temperature, chair conformation of cyclohexane can interconvert from one conformer to other very rapidly. So, after interconversion, axial groups are changed to equatorial and vice-versa.
In both the chair conformations, the methyl groups are facing towards up.