# How to draw chair-form for trans-decalin (with slight variation)?

I understand how to draw chair form for trans-decalin (figure 1) for the usual structure, but I am having difficulty if taken as shown in figure 2 wherein the hydrogens at bridge-head carbons are in opposite orientation. Please help me with relevant chair form diagram.

• See a model kit (example, for this type of question a bit too few atoms, though) if 1 and 2 are really different from each other. Mar 16, 2021 at 6:30
• I know that they are identical, since on rotation about an axis perpendicular to the plane of molecular, they are inter-convertible. But if given figure 2, translating it directly to a chair form gives the highly strained trans-decalin as shown in: masterorganicchemistry.com/2014/08/05/fused-rings/#seven Mar 16, 2021 at 7:26
• In figure 1, when we translate directly to chair form, the top bridgehead carbon can easily have the hydrogen take up the position axially pointing upwards and similarly for bottom bridgehhead hydrogen, thereby making the chair form stable. Whereas in figure 2, doing the same yields the improbable highly strained conformation. How do I resolve this issue? Mar 16, 2021 at 7:31
• I did not understand. What is wrong with drawing just the bottom left for both the structures at the top? Mar 16, 2021 at 12:50
• I need to emphasize that I am asking for a solution to paper chemistry (i.e., chemistry done on paper using 2d projections of 3d structures). I am able to visualize that the two are identical and superimposable on rotation. Using kits and models is a solution but our first instinct is to use 2d projections for simple molecules. I want to know if there is a solution to resolve the 2d projection issue I am facing. Mar 17, 2021 at 5:40

## Use of 3D models

3D version of Figure 1:

Viewed upside down (rotate about z by 180 degrees):

Now it looks like we are viewing from the bottom, so we will turn the molecule a bit around the x axis:

## Use of 2D projections

If you want to use the 2D projections, consider whether hydrogens are pointing up or down on the bridge head. First, we will interpret the chair as viewing the top of the ring from above (left panel) rather than viewing the bottom of the ring from below (right panel) to avoid ambiguity.

No matter whether you use two fused chairs as drawn by the OP (Fig 1 bottom panel) or its mirror image, the front hydrogen will point down and the back hydrogen will point up. This works well for Fig 1.

In order to use the same chair building block but show the front hydrogen pointing up, you could rotate the flat model, and connect two chairs accordingly, as shown below. In effect, you choose the location of the two bridging position to match the stereochemistry you want.

• I appreciate the 3d models Karsten but I was looking more for a solution in the realm of 2d projections as shown in question. We can use the final 3d structure that you have provided to draw the 2d projection but it looks a bit complicated to teach. Mar 17, 2021 at 5:54