Decalins

If you liked bicyclic molecules, you’ll love decalins. They are just fused bicyclic cyclohexanes

The Two Types of Decalins

Concept: How to determine the stability of a decalin.

4m
Video Transcript

Basically, declins are specific types of bicyclic molecules that can form chairs. And all they are is that they are composed of two cyclohexane rings fused together by one bond. If you think back at the types of bicyclic molecules that we studied, this would be what we called a normal bicyclic. The reason we call this one normal is simply because it does not have a bridge. Remember that normal just meant that there were two rings fused by one bond in the middle.
So what are the differences between these conformations? Because of the fact that the entire molecule is made out of just cyclohexanes put together, that means that each side can form its own conformer. And what that's done is it leads to two different types of ultimate conformers that we need to know about declins. The way that we determine the differences between both is by looking at the positions of the hydrogens in between the two rings.
So what we have is – I'll show you right now, we have trans-declin and we have cis-declin. So what's the difference here? Trans-declin is like it sounds. What it means is that the hydrogens that are attached to both sides of those bridgehead atoms – remember these atoms right here are called the bridgehead. Well, the hydrogens that are attached to the bridgehead atoms are trans to each other. What that does is it gives us a conformation where you basically have two cyclohexanes fused perfectly together and you have one H at the top face, one H at the bottom face. This is what a trans-declin would look like in a planar structure and also as a chair conformation.
Now let's look at cis-declin. Cis-declin forms when we still have those bridgehead atoms, but both of the H's go towards the same side or the same face of the rings. So what you wind up getting is you still have two chair conformations, but they're connected in a slightly different way, where now one is actually basically going to go down or up depending on how you draw it and then the hydrogens are going to go face the same direction basically. What that means is we have two different arrangements. We can either have the H's facing different directions or we can have them facing the same direction.
Cis versus trans-declin. Which of these do you think is going to be the more stable form of declin? What do you guys think? The answer is that it's going to be trans-declin because of the fact that we're going to have a lot less steric and torsional interactions in the trans position. Think about it. In this case what I have is I have two rings that are as far apart from each other as possible. I also have pretty much no torsional interaction between this H and this H. They're as far away from each other as they can possibly get so that's a really good declin.
Now let's look at cis-declin. Cis-declin, what it has is that now these rings are kind of in each other's space. One is facing the same way as before, but now one is going to be facing down and what that means is that there's going to be a little bit of interaction, oops, a little bit of interaction between these guys. On top of that now, what we're going to get is a little bit of torsional strain between those H's. So the cis-declin is actually significantly less stable than the trans-declin.
Now, what kinds of questions will your professor ask? I honestly don't know. Not all professors talk about declins. Not all professors actually care about declins as its own subject. But I have found over years of tutoring that some professors will want you guys to be able to draw these declins and to be able to say which ones more stable and which ones the least stable. 

As is hopefully evident from these drawings, trans-decalins are more stable than cis-decalins. 

Drawing Decalin

Concept: Draw the following decalin as a chair conformation in the most stable conformation.

4m
Video Transcript

Just in case your professor wants you to know about it, just in case he or she wants to throw something crazy, let's just go ahead and do this one.
So what we've got here is a declin. Notice that I've drawn the H's in a specific way, so pay attention to that. And it says please draw the declin as a chair conformation in the most stable conformation. I know this is the first time you've done this, so I'm not going to ask you guys to pause the video and then come back because it's something that I don't expect you guys to be able to do very well. Let's just do it together and then you guys will see how this works.
So first of all, which declin should I be drawing? Should I be drawing cis or trans? I should be drawing trans-declin. Why? Because I've been giving a trans-declin. But let's say that I had instead given you another declin that looked like this and it just had stick for H's. Then which way should I draw it? Should I draw it cis or should I draw it trans? The answer is that I should still draw it trans. Why? Because remember that trans is going to be the most stable as possible. The most stable possible declin. So if it's asking me for the most stable conformation, then I'm going to draw trans no matter what.
So now we've got that settled, let's go ahead and draw our trans-declin. What that's going to look like – and this is one of the hardest parts about it, just being able to draw it. You draw your chair conformation like normal. So I'm going to do this. I'm going to do that. Then what you do is you draw another chair coming directly off of that one. So I'm going to draw another stick coming off like that and then I'm just going to make the dip on the other side, so I'm going to do this and this and that.
That actually came out pretty nice. I don't always draw it that nice. It takes a little bit of practice. But there you go. We've got our trans-declin. Let's go ahead and we can draw in the H's, just so that they know what we're doing.
And now all I have to do is I have to figure out where to put that tertbutyl group. First of all, how many carbons away is it from a bridgehead? Two. The bridgehead is here, so what that means is that my tertbutyl should be right here. Now it's asking for the most stable conformation. Should I put it in the axial position or the equatorial position, which one? Equatorial.
So I'm going to put this in the equatorial position and we are done. That is the answer to this question. That is the most stable conformation of this declin.
Now there are different variations of this question. For example, your professor could give you two substituents then you have to figure out which one wants the equatorial preference. But it would still be the same principle as we learned in chair conformations that you just go with the larger group wanting the equatorial preference.
I hope that that made sense. If your professor didn't talk too much about declins, then you can feel free to skip this or not practice it so much. But I just put this in here just in case. Let's go ahead and move on.