18-C - Organic Chemistry (Mcmurry)

Question

Draw the more stable chair conformation of the following molecules, and estimate the amount of strain in each: cis-1-Bromo-4-ethylecyclohexane.

Step-by-Step Solution

Answer

The more stable chair conformation is given below and has 2 kJ/mol strain.

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.

Energy differences cause

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 conformations possible and strain calculation

                                Conformation one possible

In this image, it can be seen that  $2(\mathrm{H}\leftrightarrow \mathrm{Br})$ interactions will be present.

One interaction of $(\mathrm{H}\leftrightarrow \mathrm{Br})$ gives 1 kJ/mol energy and here two interactions are presently giving 2 kJ/mol strain.

                                    Conformation two possible

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

One interaction of $(\mathrm{H}\leftrightarrow {\mathrm{CH}}_3)$ gives 3.8 kJ/mol energy and here two interactions are presently giving 7.6 kJ/mol strain.