Ring flip and interactions in the cis decalin

There are two gauche interactions are present in the cis isomer of decalin. This interaction is responsible for the increase in energy. When doing the ring flip in cyclohexane which results that all axial group becomes equatorial and all the equatorial groups become axial.

The ring flip in the molecule of cis-decalin will leads to the conformer which is exactly equal in energy. Each of the angular hydrogen atoms are oriented axial to one of the rings and equatorial to the other ring. Such a relationship reverses if the double bond flipping converts one cis isomer into other. However, such ring flip in trans-decalin is not possible since it will lead to excessive ring strain. It can also explain in a way that the trans-decalin is locked in its position.

The stereochemistry of the carbon present in the bridgehead from a wedged H to a dashed H shall make a big difference in the overall shape of the molecule. Trans-decalin is more stable than cis-decalin. One can look at the line drawings of each of the molecule, when doing so, it was noted that in trans-decalin, all the carbons are equatorial whereas in the case of cis-decalin one carbon is axial and the other is equatorial.

Stability of the Decalin:
In the case of trans-decalin, all the carbon-carbon bonds are equatorial to each other therefore there are no gauche interactions. Hence it is more stable than cis-decalin by {\rm{10}}{\rm{.5kJ/mol}}10.5kJ/mol. It is incapable of undergoing the ring flipping due to the ring flip of one ring will reflect its two alkyl substituents to be too far apart to accommodate the second ring. There is also another reason which depicts that the cis-decalin is less stable due to the weak non-bonded interactions in the concave side. The substituent for example {\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{ = C}}{{\rm{H}}_{\rm{2}}}CH
2
​
=CH
2
​
group insufficiently long to bridge them in the case of trans-decalin. Hence the trans-decalin was found to be conformationally locked. Since it could not undergo a ring flip. The concave-concave tent-like shape of the cis-decalin the two faces are sterically non-equivalent. Therefore, the incoming substituent will prefer the less hindered convex face which is highly accessible. There are two gauche interaction occurs in the cis-decalin. The gauche interaction is the type of interaction which occurs in butane occurs if the two methyl group has the dihedral angle of {\rm{6}}{{\rm{0}}^{\rm{o}}}60
o
and {\rm{30}}{{\rm{0}}^{\rm{o}}}300
o
and arises because of both the methyl groups are quite close together. The content of high trans-decalin is reported only for the palladium catalyst that is far more trans-selective than the nickel and therefore for the hydrogenation at higher temperatures where the isomerization from the cis form takes place. However, there is no hydrogenation or isomerization was noticed under the studied conditions in which pure cis- or trans-decalin can be used as the starting material in the decane.