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Solution: Consider the structures of cis-decalin and trans-decalin: (...

Question

Consider the structures of cis-decalin and trans-decalin:

(b) One of these two compounds is incapable of ring flipping. Identify  it and explain your choice.

 

Video Transcript

Alright guys. So, for this question we have our two molecules and one at the top is going to be the one that's trans and one at the bottom is going to be our cis and it wants to know which one can't do a ring flip. Now, remember a ring flip, let's say we're starting with this molecule A here, we're going to call that chair A, if you were to flip it we'd get to this chair that looks like this, where this is our chair B. Notice that we just took this chair and flipped it around. So, now we need to accommodate the other chair, right? Because this is a fused molecule, we can't just get rid of the other ring and separate them like that, right? They're fused together what they look like if you thinking of them in a planar structure the trans one is going to look like this, right? So, in order for this six membered ring here, this chair? Well, let's call this A, right? They're the same one. Now, in order for this to accommodate another six member ring that's fused to it we're going to look at the alkyl substituents, so that's what I drew in here in black and we're going to say that the alkyl substituents are actually part of this ring, right? Those are as substituents in a fashion, right? Now notice that in order to do this ring flip they would have to be oriented in this direction because before they were equatorial, now they're going to be axial right? in our ring flip if you're thinking of them as substituents, so now, do you think a ring could be accommodated where our ring has to take up this space of our molecule? Well, probably not, right? Because, let's take a look at our cis one. Now, for our cis one this wouldn't have a problem because notice the way it's oriented right now so the one that's not going to be able to be the ring slip is actually our trend, the trans is going to be more stable than this one, so it's definitely not going to want to do a ring flip because look at the conformation they would have to rely on, right? It could only exist in this fashion right here. So, for that reason, we're going to say that our trans is going to be the one that can accommodate a ring flip because in order to do so it would have to have a ring, take up this space of the monitor right here, which we know is not going to be possible. Alright guys, so hopefully this makes sense and now let's take a look at your next question.