Decalin - Online Tutor, Practice Problems & Exam Prep

Declins are bicyclic molecules that can exist in two conformations: trans and cis. In trans declin, the hydrogens on the bridgehead atoms are opposite each other, leading to greater stability due to reduced steric and torsional strain. Conversely, cis declin has hydrogens on the same side, resulting in increased steric interactions and lower stability. When drawing the most stable conformation, always opt for trans declin, placing larger substituents in equatorial positions to minimize steric hindrance.

How to determine the stability of a declin.

So, basically, declins are specific types of bicyclic molecules that can form chairs. And all they are is that they are molecules that we have studied; this would be what we called a normal bicyclic. And the reason we call this one normal is simply because it does not have a bridge. Remember, normal just meant that there were 2 rings fused by 1 bond in the middle.

So what are the differences between these conformations? Well, because of the fact that the entire molecule is made out of just cyclohexanes put together, that means each side can form its own conformer. Alright? And what that's done is it leads to 2 different types of ultimate conformers that we need to know about declins. And 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? Well, trans declin is like it sounds. What it means is that the hydrogens that are attached to both sides of those bridgehead atoms, remember those are these atoms right here are called the bridgehead. Okay? 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 2 cyclohexanes fused perfectly together and you have 1 H at the top face, 1 H at the bottom face. So 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 we wind up getting is you still have 2 chair conformations but they're connected in a slightly different way. Where now one is 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. So what that means is we have 2 different arrangements. We could 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? And 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 transposition. Think about it. In this case, what I have is I have 2 rings that are as far apart from each other as possible. Additionally, I also have pretty much no torsional interaction between this H and this H; they are 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. Additionally, now what we're going to get is a little bit of torsional strain between those H's. So the cis declin is significantly less stable than the trans declin.

Now what kind 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. However, I have found over the years of tutoring that some professors will want you guys to be able to draw these declins and to be able to say which one is more stable and which one is least stable.

Draw the following declin as a chair conformation in the most stable conformation.

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. Alright? So what we've got here is a decalin, and notice that I've drawn the H's in a specific way, so pay attention to that. It says, please draw the decalin as a chair conformation in the most stable conformation. Okay? I know this is the first time we'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 decalin should I be drawing? Should I be drawing cis or trans? I should be drawing trans decalin. Why? Because I've been given a trans decalin. But let's say that I had instead given you another decalin that looks like this and it just had sticks for ages. 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 decalin. So if it's asking me for the most stable conformation, then I'm going to draw a trans no matter what. So now we've got that settled, let's go ahead and draw our trans decalin. What that's going to look like, 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. Okay. Then what you do is you draw another chair coming directly off of that one. Okay? 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. And 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 decalin. 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 tert-butyl group. So first of all, how many carbons away is it from a bridgehead? Two. Okay. The bridgehead is here, so what that means is that my tert-butyl should be right here. Okay. So now it's asking for the most stable conformation. Should I put it in the axial position or the equatorial position? Which one? Equatorial. Okay. So I'm going to put this in the equatorial position and we are done. Okay. That is the answer to this question. That is the most stable conformation of this decalin. Okay. Now there are different variations of this question. For example, a professor could give you two substituents, then you have to figure out which one wants the equatorial preference. Okay? 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. So I hope 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. Okay? But I just put this in here just in case. Alright? So let's go ahead and move on.

Here’s what students ask on this topic:

What are the differences between cis and trans decalin?

Cis and trans decalin are two conformations of the bicyclic molecule decalin. In trans decalin, the hydrogens on the bridgehead atoms are on opposite sides, leading to greater stability due to reduced steric and torsional strain. This means the two cyclohexane rings are fused in a way that minimizes interactions between the hydrogens. Conversely, in cis decalin, the hydrogens on the bridgehead atoms are on the same side, resulting in increased steric interactions and lower stability. The rings in cis decalin are closer together, causing more strain and making it less stable than trans decalin.

Why is trans decalin more stable than cis decalin?

Trans decalin is more stable than cis decalin because the hydrogens on the bridgehead atoms are on opposite sides, which minimizes steric and torsional strain. In this conformation, the two cyclohexane rings are as far apart as possible, reducing interactions between the hydrogens and other atoms. This spatial arrangement leads to less steric hindrance and torsional strain, making trans decalin more stable. In contrast, cis decalin has the hydrogens on the same side, causing the rings to be closer together and increasing steric interactions, which results in lower stability.

How do you draw the most stable conformation of decalin?

To draw the most stable conformation of decalin, you should draw the trans decalin. Start by drawing two chair conformations of cyclohexane fused together. Ensure that the hydrogens on the bridgehead atoms are on opposite sides. If there are substituents, place the larger substituent in the equatorial position to minimize steric hindrance. This arrangement reduces steric and torsional strain, making the conformation more stable. Practice drawing the chair conformations accurately to ensure the correct spatial arrangement.

What is the significance of bridgehead atoms in decalin?

Bridgehead atoms in decalin are the carbon atoms where the two cyclohexane rings are fused together. These atoms are crucial in determining the conformation and stability of the molecule. The positions of the hydrogens attached to the bridgehead atoms dictate whether the decalin is in the cis or trans conformation. In trans decalin, the hydrogens are on opposite sides, leading to greater stability. In cis decalin, the hydrogens are on the same side, resulting in increased steric interactions and lower stability. Understanding the role of bridgehead atoms helps in predicting the stability and drawing the correct conformation of decalin.

How do steric and torsional strain affect the stability of decalin conformations?

Steric and torsional strain significantly affect the stability of decalin conformations. Steric strain arises from the repulsion between atoms or groups that are too close to each other, while torsional strain results from eclipsing interactions between bonds. In trans decalin, the hydrogens on the bridgehead atoms are on opposite sides, minimizing both steric and torsional strain. This makes trans decalin more stable. In contrast, cis decalin has the hydrogens on the same side, leading to increased steric interactions and torsional strain, which decreases its stability. Therefore, reducing these strains is key to achieving a more stable conformation.