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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.

chair-form for trans-decalin

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  • $\begingroup$ 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. $\endgroup$
    – Buttonwood
    Mar 16, 2021 at 6:30
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    $\begingroup$ 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 $\endgroup$ Mar 16, 2021 at 7:26
  • $\begingroup$ 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? $\endgroup$ Mar 16, 2021 at 7:31
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    $\begingroup$ I did not understand. What is wrong with drawing just the bottom left for both the structures at the top? $\endgroup$
    – Alchimista
    Mar 16, 2021 at 12:50
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    $\begingroup$ 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. $\endgroup$ Mar 17, 2021 at 5:40

2 Answers 2

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Use of 3D models

3D version of Figure 1:

enter image description here

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

enter image description here

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

enter image description here

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.

enter image description here

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.

enter image description here

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    $\begingroup$ 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. $\endgroup$ Mar 17, 2021 at 5:54
  • $\begingroup$ @Ritwik_Kumar I added a second part to my answer. $\endgroup$
    – Karsten
    Mar 17, 2021 at 12:52
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    $\begingroup$ Because the 2D images in the original post are identical, the 3D image shown fits either one. The reason this image is usually drawn is that the methine hydrogens are not visually obscured by other bonds. See Karsten Theis's final two images with methine hydrogens in place. $\endgroup$
    – user55119
    Mar 17, 2021 at 15:18
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A common notation in crystallography used for denoting atoms above and below the plane of the paper for the structures with quasi-flat overall shape is the use of filled (⚫) and open (⚪) circles, respectively. I think you can use this technique for decalin as well. For trans-decalin with both rings in chair conformation there is even no need to change your drawing for pseudo-3D (red plane refers to the plane of drawing, not symmetry) (scheme A).

Also, note that in crystallography there are also plus (+) and minus (−) signs used for the same purpose, e.g. denoting an object (including atoms) that lies above and below the plane containing the reference axes, respectively. I personally find this notation less applicable here because of the possible confusion arising with the clash of notation for charge distribution (scheme B).

depiction of trans-decalin in 2D with inconvenient H-placement point of view

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