Q4-23E - Organic Chemistry (Mcmurry)

Question

Name the following compound, identify each substituent as axial or equatorial, and tell whether the conformation shown is the more stable or less stable chair form

 

Step-by-Step Solution

Answer

The Methyl group is present at the axial position, and chlorine is present at the equatorial position. Given structure is less stable chair conformation.

Definition of Chair confirmation

Chair conformation structure for the six-membered ring is when atoms 2, 3, 5, and 6 lie in the same plane, atom 1 lies above the plane, and atom 4 lies below the plane.

Explanations of Axial and equatorial positions

Axial bonds are the bonds that are perpendicular or vertical to the ring plane. Equatorial bonds are approximately horizontal in the plane of the ring. 

Structure stability

The energy difference between axial and equatorial conformations is due to steric strain caused by 1,3-diaxial interactions. The axial group on C1 is too close to the axial hydrogens, three carbons away on C3 and C5. 

Stability of given structure

Here methyl group is present vertically in the ring, indicating it is at the axial position, and chlorine is present approximately horizontal in the plane of the ring, indicating its equatorial position.

Each methyl group gives two interactions, creating a 3.8 kJ/mol strain, hence a total strain of 7.6 kJ/mol.

Stability of other structure possible

If the structured group is present vertically in the ring, at the axial position, and a methyl group is current approximately horizontal in the plane of the ring, at the equatorial position, then:

In this case, the chlorine will form two interactions with hydrogen. Each creates a strain of 1 kJ/mol, and hence total strain of 2 kJ/mol is less than the given structure and therefore is more stable than the given structure.