Chapter 4, Problem 4.9P

Interpretation Introduction

Interpretation:

The two chair conformations of ${\text{cyclohexane-d}}_{\text{1}}$ in which one hydrogen atom has been replaced by a deuterium atom are to be drawn. Which of the two conformers is expected to be present in greater abundance is to be determined.

Concept introduction:

Cyclohexane exists in two chair conformations which interconvert through a chair flip. The hydrogen atoms in cyclohexane are either in axial or equatorial positions. During chair flip, the hydrogens that were in an axial position are in the equatorial position and vice versa. However, the two chair conformers of cyclohexane are indistinguishable as all atoms bonded to the ring carbons are small hydrogen atoms.

In substituted cyclohexane, the larger size of the substituent atom or group reduces the stability of one of the conformations. Bulky groups require more space than hydrogen atoms and, therefore, are more stable in the equatorial position. A substituent in axial position is close to the two hydrogen atoms in axial positions on the same side of the ring. There are no such hydrogens in close proximity to a substituent in an equatorial position. This makes the equatorial conformer more stable.