Question

Given the planar trisubstituted cyclohexane below, fill in all boxes (with H or Cl) to complete the two possible cyclohexane chair conformations.

Answer 1

Concepts and reason

The stability can be easily determined using the chair conformation. The stereochemistry of the substituents on a cyclohexane determines their position in the chain conformation of cyclohexane that is axial or equatorial.

Fundamentals

Conversion of cyclohexane to its respective chair conformation is totally based on the stereochemistry of the substituents in cyclohexane. The rules for indicating the positions of the substituents inthe chair conformation are as follows:

1.Trans-1,3- substituted substituent in cyclohexane occupy either axial and equatorial or equatorial and axial positions in achair conformation.

2.The cis-1,3-substituted substituent in cyclohexane occupies either axial and axial or equatorial and equatorialposition in achair conformation.

The bulky group in the equatorial position is more stable than in axial position. The bulky group in the axial position will have a1,3-diaxial interaction which raises the energy of the conformation and hence, is less stable. But groups attached to equatorial positions will be placed apart and hence, there will be no steric hindrance. Therefore, they have low energy conformation which is more stable.

Ring flipping or chair flip of one conformation gives another conformation. Ring flipping interconverts axial and equatorial positions. The ring flipping phenomenon has same connectivity in both conformations but has different shapes.

The given cyclohexane is as follows:

All three chlorine groups are cis to each other. And the relationship between all three chlorines is 1,3-substituted.

The chair 1 conformation with the chlorine and hydrogen atoms filled is as follows:

Ring flipping of chair 1 conformation is as follows:

The stable conformation is chair 2.

Ans:

The two possible cyclohexane chair conformations are as follows:

The more stable conformer is: