When there are two or more substituents on the ring, stereoisomerism is possible. For example, there are two isomers of 1,4-cyclohexanediol—in one (the cis isomer) both the substitu ents are either above or below the cyclohexane ring; in the other (the trans isomer) one hydroxyl group is above the ring whilst the second is below. For a cis 1,4-disubstituted cyclohexane with both the substituents the same, ring inversion leads to a second identical conformation, while for the trans configuration there is one conformation with both groups axial and one with both groups equatorial.

The chair-structure diagrams contain much more information than the simple ‘hexagon’ diagrams that we have used up to now. The former show both configuration and conformation— they show which stereoisomer (cis or trans) we are talking about and also (for the trans com pound) the conformation adopted (diaxial or the more stable diequatorial). In contrast, the simpler hexagon diagrams carry no information about the conformation—only information about which isomer we are dealing with. This can be useful because it enables us to talk about one configuration of a compound without specifying the conformation. When you are solving a problem requiring conformational diagrams to predict the configuration of a product, always start and finish with a configurational (hexagon) drawing. The chair conformer of cis-1,4-disubstituted cyclohexane has one substituent equatorial, the other axial. This will not necessarily be the case for other substitution patterns, for exam ple the chair conformer of a cis-1,3-disubstituted cyclohexane has either both substituents axial or both equatorial. Remember, the ‘cis’ and ‘trans’ prefixes merely indicate that both groups are on the same ‘side’ of the cyclohexane ring. Whether the substituents are both axial/equatorial or one axial and the other equatorial depends on the substitution pattern. Each time you meet a molecule, draw the conformation or make a model to find out which bonds are axial and which are equatorial.

What if the two substituents on the ring are different? For the cis-1,3-disubstituted example above, there is no problem because the favoured conformation will still be the one that places these two different substituents equatorial. But when one substituent is axial and the other equatorial (as they happen to be in the trans diastereoisomer above) the preferred con formation will depend on what those substituents are. In general, the favoured conformation will place the maximum number of substituents equatorial. If both conformations have the same number of equatorial substituents, the one with the larger substituent equatorial will win out, and the smaller group will be forced to be axial. Various possibilities are included in the examples below.

This is only a guideline, and in many cases, it is not easy to be sure. Instead of concerning ourselves with these uncertainties, we shall move on to some differentially substituted cyclohexanes for which it is absolutely certain which conformer is preferred.

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

Decalins

Substituted cyclohexanes

The ring inversion (flipping) of cyclohexane

A closer look at cyclohexane