Question

Below are isomers of 1,2-di(2-methylpropyl)cyclohexane in conformational structures. Two are cis and two trans. Indicate which is the more stable conformation for the cis and which for the trans, and indicate which of all the conformations is the most stable.

If both conformations of the isomer are equally stable, select both.

Select the mote stable conformation of cis-1,2-di(2-methylpropyl)cydohexane.

Select the more stable conformation of trans-1,2-di(2-methy1propyl)cyclohexane.

Select the most stable conformation overall.

Answer 1

Concepts 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.

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.

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.

The structures of cis-1,2-di(2-methylpropyl)cyclohexane is as follows:

The larger substituents that are in equatorial position will be more stable.

The structures of trans-1,2-di(2-methylpropyl)cyclohexane is as follows:

The larger substituents that are in equatorial position will be more stable.

The four conformers are as follows:

Ans:

The more stable conformers are A and C as they are equally stable. Both the conformers contain the larger substituent at the equatorial position.

The more stable conformer is B because both the larger substituents in equatorial position.

Thus, among the four conformers (A, B, C and D), B is more stable.