Conformations of cyclohexane
How to analyze the chair and boat conformations of cyclohexane.
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- what is the difference between torsional strain and eclipse?(5 votes)
- There is no difference.
Torsional strain is the strain caused by eclipsed bonds.(17 votes)
- What is the difference between angle strain, torsional strain, and steric strain? I did not entirely understand those concepts, thanks!(6 votes)
- Angle strain - an angle deviates from, ideally, 109.5 degrees, and there's an energy increase.
Steric strain/ steric hindrance - bulky groups of substituents repelling each other.
Torsional strain - when bonds repel, but there doesn't have to be bulky groups. An example would be bonds near each other in an eclipsed conformation.
You could also check out "Stability of cycloalkanes" and "Stabilization of a conjugate base: salvation" videos in the organic chemistry section for more details!(7 votes)
- Why don't you draw in the ch2 molecules on the neuman projections? At8:21, for example, and I think it also occurs in the chair conformation drawing somewhere.(2 votes)
- There are not CH2 groups in the molecule, only hydrogen atoms. However, I don't know if there is a reason that the instructor left off the hydrogen atoms.(2 votes)
- The subtitles in this video call the molecule cyclohexene, however I think it's cyclohexane because it only has single bonds, right? I'm not sure which he pronounced.(0 votes)
- You are right, it is cyclohexane! Not cyclohexene. The subtitles are a little off sometimes.(4 votes)
- How do you pick which carbon is carbon 1?(1 vote)
- On cyclohexane, it really is arbitrary because all carbons can rotate. If there's a substituent (methyl, etc) then the carbon the substituent is connected to becomes C-1.(1 vote)
- what is the angle strain for cyclobutane?(1 vote)
- * Torsional strain is caused due to eclipsed conformation.
* Angle strain is due to small or grater bond angle than normal.(1 vote)
- At4:38, Jay talks about flagpole interactions. Is that steric interaction?
Also, does the statement, "Hydrogen-hydrogen diaxial interactions won't cause any strain" only apply to (axial) hydrogens in the chair conformation? What I understand is that such an interaction forms the baseline, and all other interactions (including twist boat interactions?) create relatively more potential energy.(1 vote)
- what is the order of stability of conformations of cyclohexane(1 vote)
- In the order form the most to least stable, Chair, Twist Boat, Boat and finally half-chair.(1 vote)
- For cis-1,3-dimethylcyclohexane, which two chair conformations are in equilibrium?(1 vote)
- For cis-1,3-dimethylcyclohexane, which two chair conformations are in equilibrium?(1 vote)
- [Voiceover] Here we have a model of the cyclohexane molecule and it looks like it's a flat hexagon from this perspective, but it isn't really. If we turn it to the side we can see this is not a planar molecule. This is called the chair conformation of cyclohexane. Now if we stare down these two carbines, we'll be able to see the chair conformation from a Newman projection viewpoint, so now you can see that we have staggered hydrogens. Here we have a picture of the chair conformation from the video and the reason why we call it the chair conformation is if you tilt it on its side a little bit, it looks a little bit like a chair, so it has these three parts to it essentially, you have a place for your back and your head, so let's say that's this area right here, there is a seat right here, and then finally there's a foot rest, so because it looks a little bit like a chair, that's why we we call this the chair conformation. Here's how to draw the chair conformation and let's compare this drawing to the picture. On the picture we'll start with this carbon right here which is actually carbon one and that's this carbon here and this carbon has two hydrogens, so there's the two hydrogens. Next we have this bond in here which is represented by this bond which takes us to carbon two. Carbon two also has two hydrogens. Of course every carbon in cyclohexane has two hydrogens on it. Next we go to this bond right here which is represented right here on our chair conformation and we draw in our two hydrogens. Our next bond goes down a little bit in this direction, so that's this bond and then we get to this carbon and we put in our two hydrogens. Our next bond goes back like this, so here's this bond and then we'll put in our two hydrogens on this carbon. Then this bond is back here, you can see it on the picture and that's this bond that goes behind this front hydrogen here. Then this carbon has two hydrogens, so we'll draw those in. Then finally this last bond here goes up a little bit in space, so this one goes up like that, we're back to carbon one. Now let's look at the Newman projection, right, so this is what we saw in the video, let's go ahead and draw that. We'll start with this carbon right here, so we will represent that with a point. We can see there's a hydrogen going straight up, so let's draw in our hydrogen going straight up, we have a hydrogen going down and to the left, so a hydrogen going down and to the left, and then we have down and to the right we have a CH2, so I won't draw in CH2, we'll just know that's there. For the back carbon you can actually see a little bit of the back carbon in the picture just because of the perspective, but the front carbon should be completely eclipsing the back carbon so we represent that with our circle. Then what's coming off of the back carbon, well there's this hydrogen, so we'll draw in that hydrogen like that, so going up and to the left. There's a hydrogen going down, so we'll draw that hydrogen in here. Then we have going to the right and up we have a CH2, so going to the right and up is a CH2 group. Move over to this carbon over here on the right so we represent that with a point, so we'll draw that in. Hydrogen going straight up on that carbon, so we'll draw in that hydrogen. We have a hydrogen going down and to the right, so we draw in that one and then we have this bond connecting to that CH2 group, so we can draw that in here like that. Then we have our back carbon which we can't see in the picture, but we know there's a back carbon here and bonding to the back carbon is a hydrogen going up and to the right, so we draw that one in. A hydrogen going straight down, so we draw that one in, and then finally we have, let me go ahead and correct that one a little bit, so we have a hydrogen going straight down. Then finally we have a bond to the back carbon going to this CH2 group, so we can draw that in here like this. Now we have a Newman projection for the chair conformation and the nice thing about the newman projection is it shows you your hydrogens are all staggered here, so we have staggered hydrogens, so we don't have any torsional strain to worry about and with a chair conformation the bond angles are pretty close to the ideal bonding, a little over 109.5 degrees, so the carbon carbon carbon bond angles are approximately 111 degrees, so we don't have to worry about any angle strain. We don't have to worry about any torsional strain, so the chair conformation is the most stable conformation for cyclohexane. Next we'll take a look at the boat conformation. Here we have the boat conformation of cyclohexane, if you look at the carbons it looks a little bit like a boat. There are a few things that destabilize the boat conformation and one of them is these top hydrogens here, so they're close enough to get in the way of each other and that's called flag pole interaction and that increases the strain. If we look down at these two carbons, the one in the back and the one in the front here, we'll see the boat conformation from a Newman projection viewpoint, so now we're looking at it from a Newman projection perspective. We can see there's another source of strain, there's torsional strain. For example we have these front hydrogens here eclipsing these other hydrogens and there's lots of examples of torsional strain destabilizing the boat conformation. Here's the boat conformation from the video. Let's analyze that and compare it to the drawing up here, so this bond right here is this bond, I made it much longer on the drawing just to make it easier to see the hydrogens. This carbon right here would be this one. We'll draw in those two hydrogens. Then we have a bond going up to this carbon, so that's our bond going up right here and then we have two hydrogens bonded to that carbon. Then this bond goes down and back to our carbon back here, this one, and then there are two hydrogens on this carbon, so here and here. Hard to see those hydrogens on this carbon in the picture, but there's one. Then the other one there's the tip of the other one sticking out. Then we have this bond back here which is this one and we'll draw on our hydrogens here as well. Then we go back up to this carbon, so we're going up to this carbon, draw in our two hydrogens on that carbon, and then we go back down to here and that's this carbon on the drawing, we'll put in our two hydrogens. We can see the flag pole interaction so when this hydrogen, when this hydrogen gets too close to this hydrogen, when they get in the way of each other that increases our strain. We also have this little boat shape here, so hopefully you can see, let me change colors here, so you can see how this looks like a boat if you just look at your carbons. We have a boat like that. Because this is in a boat conformation, there's not really much in the way of angle strain, but we know there's some torsional strain and the best way to see that is with our Newman projections, so next look over here for our Newman projection. Let's draw what we see and we'll start with this carbon so that's represented by a point. Then we have a hydrogen going up and to the left, that's this one right here. Then we have a hydrogen going down, so a hydrogen going down like that. Then up and to the right we have a bond to a CH2, so up and to the right is our bond to a CH2, make that a little bit longer. Then there's a back carbon, so I draw in a circle to represent the back carbon. You can see the hydrogens in the back carbon are eclipsed by the hydrogens in the front carbon, so this hydrogen right here is pretty much eclipsed, I'll draw it a little bit out so we can still see it. Then this one is a little bit down, so we'll draw that one like this. Then the bond in the back, it's very hard to see but we know there's a bond back there going to another CH2 group, so I'll draw that back here like this. Next we look at this carbon, so we represent that with a point. We're not really looking straight down this axis like we were on this situation, so I'll just treat the right the same way that we treated the left sides. We have a hydrogen going up and to the right. We would have a hydrogen going down like this and then we would have this bond going to this CH2 group, so we have to show this bond meeting up with this one right here, so this is CH2 right here. Then the back carbon would be a circle, so we have a hydrogen going up and to the right, we have a hydrogen going down, and then finally we would have a bond from that back carbon going to the other CH2, so going to this other CH2 here. This allows us to see all the eclipsed hydrogens and all of the torsional strain, so the boat conformation is much higher in energy compared to the chair conformation, the chair conformation is the lowest in energy. There are actually, there are other conformations of cyclohexane, so the boat conformation can actually twist a little bit to give you twist boat. There's also half chair, but we're not really too concerned with those other conformations and actually we're gonna focus on the chair conformation in future videos because cyclohexane spends most of its time in the chair conformation.