4-18-D - Organic Chemistry (Mcmurry)

Question

Draw the more stable chair conformation of the following molecules, and estimate the amount of strain in each: cis-1-tert-Butyl-4-ethylcyclohexane

Step-by-Step Solution

Answer

The more stable chair conformation is below and has an 8 kJ/mol strain.

Chair conformation

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.

Axial and equitorial positions

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

Cause for energy difference

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. 

Possible chair conformations and strain calculation

                            Conformation one possible

In this image, it can be seen that $(\mathrm{H}\leftrightarrow {\mathrm{CH}}_2{\mathrm{CH}}_3) 2(\mathrm{H}\leftrightarrow {\mathrm{CH}}_2{\mathrm{CH}}_3)$  interactions are present.

One interaction gives 4 kJ/mol energy, and here two interactions are presently giving 8 kJ/mol strain.

                              Conformation two possible

In this image, it can be seen that tertiary butane is present here. Hence it has $(\mathrm{H}\leftrightarrow \mathrm{C}{\left({\mathrm{CH}}_3\right)}_3) 2(\mathrm{H}\leftrightarrow \mathrm{C}{\left({\mathrm{CH}}_3\right)}_3)$ interaction present.

One interaction gives 11.4 kJ/mol energy, and two interactions presently give 22.8 kJ/mol strain.

More stable chair configuration

Hence, it can be seen this structure has less strain and is, therefore, more stable.

                        Conformation one is more stable