It is also possible to lock the conformation of a cyclohexane ring by joining another ring to it. Decalin is two cyclohexane rings fused at a common C–C bond. Two diastereoisomers are possible, depending on whether the hydrogen atoms at the ring junction are cis or trans. For cis-decalin, the second ring has to join the fi rst so that it is axial at one point of attachment and equatorial at the other; for trans-decalin, the second ring can be joined to the fi rst in the equatorial position at both attachment points.

When a cyclohexane ring inverts, the substituents that were equatorial become axial and vice versa. This is fine for cis-decalin, which has an axial–equatorial junction, but it means that ring inversion is not possible for trans-decalin. For trans-decalin to invert, the junction would have to become axial–axial, and it’s not possible to link the axial positions to form a six-membered ring. Cis-decalin, on the other hand, ring inverts easily.

Flipping cis-decalin: not so difficult once you know how

 If you find it hard to visualize the ring inversion of cis-decalin, you are not alone! The best way to think about it is to ignore the second ring until the very end: just concentrate on what happens to one ring (black in this diagram), the hydrogens at the ring junction, and the (orange) bonds next to these hydrogens that form the ‘stumps’ of the second ring. Flip the black ring, and the ‘stumps’, and the hydrogens swap from axial to equatorial and vice versa. Draw the result, but don’t fi ll in the second ring yet or it will usually just come out looking like a flat hexagon (as in diagram A). Instead, rotate the complete (black) ring 60° about a vertical axis so that both of the orange ‘stumps’ can form part of a chair, which can now be fi lled in (diagram B). To make a chair (and not a hexagon) they must be pointing in a convergent direction, as the orange bonds are in B but not A.

مواضيع ذات صلة

E1 reactions can be stereoselective

The role of the leaving group

Substrate structure may allow E1

E1 and E2 mechanisms

Size

Elimination happens when the nucleophile attacks hydrogen instead of carbon

Axially and equatorially substituted rings react differently

Steroid

What happens with more than one substituent on the ring

Substituted cyclohexanes

The ring inversion (flipping) of cyclohexane

A closer look at cyclohexane