Chapter 26.6, Problem 20CC

Interpretation Introduction

Interpretation:

Hypothetical ring-flip of trans decalin has to be drawn and the reason why it does not occur need to be explained.

Concept introduction:

Ring-flipping is a phenomenon known as ring inversion that involves rotation about the single bonds of cyclic conformers.  Usually this happens in cyclohexane ring.  When ring flip happens in the cyclohexane, the axially and equatorially substituted groups are inverted.  The ring-flipping also happens in a fused ring system.  Decalin is a fused ring system.  There are two stereoisomers for decalin, namely cis-decalin and trans-decalin.

To draw and explain: hypothetical ring-flip of trans-decalin does not occur and why cholesterol has a fairly rigid 3D geometry.