trans-decalin

"Flipped" trans-decalin

Trans-decalin, where two cyclohexane rings are "fused" in the middle, share two common carbon atoms. The most stable conformation of this molecule has both rings in a chair conformation. As the name implies, the substituents on the cyclohexane ring are trans. For sake of discussion we will consider the left hand ring as the original cyclohexane and the right hand ring as the substituents. Because the molecule is symmetrical we can actually start from either ring. Select the button for trans-decalin which shows the equatorial carbons in red. Even though the carbons are both equatorial, they are still transas the carbon in front points upwards and the carbon in the back points downwards. Remember, each carbon will have one atom attached that points up and one that points down. What is interesting about trans-decalin is that it cannot undergo a "ring flip.". In order to do a "ring flip" the two equatorial carbons would have to become axial. The consequences are shown on the "flipped" trans-decalin. Select the button that shows their flipped position. It is not difficult to see that this creates a ridiculously long carbon-carbon bond. It is 3X the length of a normal carbon-carbon bond and cannot exist. Therefore, the molecule is unable to adopt this conformation. If this structure is put into a computer and the energy minimized the axial carbons go back to the equatorial positon. Thus, trans-decalin maintains its intital conformation around the ceter two carbons with substituent carbons equatorial, unable to flip. Likewise, the two hydrogens that are common to both rings are fixed. They are in trans diaxaial orientation, anti to each other. Any substituents replacing the cetral hydrogens would be locked in an trans diaxial conformation, anti to each other.
There are more models and a good discussion at the Yale website: here.