Q4-16P - Organic Chemistry (Mcmurry)

Question

Why do you suppose an axial cyano (–CN) substituent causes practically no 1,3-diaxial steric strain (0.4 kJ/mol)? Use molecular models to help with your answer.

Step-by-Step Solution

Answer

An axial cyano substituent causes practically no steric strain because the cyano group is presently pointing straight up.

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.

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 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. 

Here, the cyano group is present because the carbon is sp hybridized.

Molecular model

A molecular model with cyano substituent shown in pink.