4-19 - Organic Chemistry (Mcmurry)

Question

Identify each substituent in the following compound as axial or equatorial, and tell whether the conformation shown is the more stable or less stable chair form (green = Cl) :

Step-by-Step Solution

Answer

The given structure 1-chloro-2,4-dimethyl-cyclohexane . In this structure, the chlorine and methyl group on carbon four are axial, and carbon two methyl groups are equatorial. This structure is less stable.

Chair conformation

Chair conformation structure for a 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.

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. 

Cause for energy differences

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, resulting in the steric strain. 

Similarly, the axial group on C2 is too close to the axial hydrogens three carbons away on C4 and C6, resulting in the steric strain. 

Chair conformation stability

Here, two conformations  are possible one is axial, and the other is equatorial. The image given methyl and chlorine group are shown at the axial position. So, here two $(\mathrm{H}\leftrightarrow {\mathrm{CH}}_3)$interactions are present, and two $(\mathrm{H}\leftrightarrow \mathrm{Cl})$ interactions are present, making it more strained. In equatorial form, there will be less steric strain and, hence, more stable as there are no such unfavoring interactions.