4.6 Cycloalkanes and Cyclohexane Chair Conformations | Organic Chemistry

cycle alkanes that will be the topic of this lesson in my

organic

chemistry

playlist and, more specifically, we are talking about

cyclohexane

more than any other cycle alkanes and, in addition, specifically the

chair

conformations

of

cyclohexane

like Well , this comes immediately after an entire chapter on alkanes. We spent the first half of the chapter explaining how to name alkanes and then we've been talking about different confirmations of alkanes. We went through Newman projections in the last lesson and this. The lesson will talk about cycloalkane confirmations and again, most importantly, cyclohexane confirmations. Now if you're new to the channel, my name is Chad and welcome to Chad's Prep.

My goal is simply to make science understandable and even enjoyable now. is my new

organic

chemistry

playlist. I will be posting these lessons weekly during the 2020-21 school year, so if you don't want to miss any, subscribe to the channel, click the notification bell, you will receive a notification every time I post one. new video so

cycloalkanes

and here we have cyclopropane cyclobutane cyclopentane cyclohexane and cycloheptane on the board and first we want to take a look at cyclopropane and cyclopropane adopts the conformation of an equilateral triangle here at least for the carbon atoms so let's draw a couple of relevant hydrogens here, so we have one here and here and here and here and as a reminder, the wedge bonds here, I mean these hydrogens are coming off the board, the dash bonds mean the hydrogen is going on the board, so I The most important thing is What I want to see is the bond angles inside the triangle here inside that equilateral triangle and so for the nickel outside the triangle, those bond angles are 60 degrees and that's a problem, so that all the carbons are here with sp3 hybridization and, having sp3 hybridization, they want to adopt.

The bond angles of 109.5 and 60 degrees are not even close, and it turns out that when your angle is much lower than what it wants to be, that is associated with what we call cold angular stress and cyclopropane has a ton of angular stress now if you observe. cyclobutane cyclobutane is pretty much like a square, it turns out that it bends, so it's not perfectly flat, it's not perfectly two-dimensional, so these angles aren't exactly 90 degrees, they're like 88 degrees, but close enough to which in this case has a little bit less of this angular stress associated with it now the angular stress is just one part, it's one of the three components of a larger issue that we call annular stress and there are two other parts of this annular stress and There are two things that you have already visited before one is the steric tension atoms colliding with each other and the other is the torsional tension repulsion between the electrons in the bonds and, as you remember, we studied this in the last lesson on Newman projections, we learned that eclipse

conformations

have significantly more steric and torsional strains than staggered ones. conformations one of the reasons they have higher energy, well, it turns out that here in cyclopropane, so if you look at this carbon-carbon bond here, you'll find that these two hydrogens line up perfectly and these two hydrogens line up perfectly and it's a eclipsed conformation and therefore cyclopropane not only has a good amount of angular strain, but it has a good amount of both steric and torsional strain and the combination of the three we would say that cyclopropane has a good amount of annular strain, in fact.

It has more ring tension than any of the other

cycloalkanes

, and a lot of that comes down to that angle tension. Now, if we look at cyclobutane, that angle is still not 109.5, but it's much closer than this, so it's cyclopropane. In fact, it even has bent bonds when the orbitals overlap to form those carbons instead of being dead, so they are a little bit at an angle and we say they have bent bonds and it turns out that these bonds are actually much weaker than in the other structures. and cyclopropane will carry out certain chemical reactions that under this other cycle the alkanes and so on will do, so it is so unstable that, as I said, it has a certain chemical reactivity associated with it that the others do not work well, so it turns out that at that moment you get to cyclohexane here, cyclohexane has the ability in its

chair

conformations to have zero ring tension, but it turns out that as you get smaller and smaller you will have more and more of this ring tension. associated with this, but it turns out that once you get past six carbons as well, now all of a sudden you can't get perfect bond angles, you can't get 109.5 degree angles exactly like you could in the chair conformations of cyclohexane. , etc. you get more than six carbons in your ring and you also start to find tension in the ring.

Now we won't look at bigger too often, but we will see this trend that the smaller the ring, the greater the amount of tension in the ring and we get A little technical here, we say that there is more tension in the ring per carbon atom , that's how we generally express it, so cyclopropane has the most ring stress per carbon atom in the ring, so because cyclohexane is the most stable by far, therefore it's the The most common ring by far turns out to be the nature and things of a story and it's the one we're going to study more with the rest of this lesson.

Now it turns out that cyclopropane is also quite common in nature's five-membered rings. but it has a little bit of ring stress, but once you get to the cyclobutane, you have quite a bit of ring stress and then you cycle the propane with a ton of ring stress associated with it, so now we're going to go through the whole run of the lesson talks specifically about cyclohexane now it turns out that cyclohexane can actually take on a few different conformations and the chair conformation is going to be the most important one and the one that we're going to spend pretty much the rest of the lesson studying, but I just want to briefly mention the other confirmations and the other one is a ship, but it has higher energy than any of these chair conformations and this ship confirmation has a couple of hydrants that we often like to identify called flagpole hydrants and if you start replacing these flagpole hydrants flag Hydrogens with larger groups will hysterically hinder each other more and more and things like that, but it's one of the reasons why this is a higher energy confirmation than these chair conformations.

Now it turns out that there are also things called spin ships where this just twists some of the angles a little bit and things like that and the only thing you really need to know is that cyclohexane has other confirmations besides the ones that we're going to spend the whole lesson studying and that They call bounces or turns. ships and they have higher energy and so there's not a lot of them actually present in solution when we start looking at cyclohexane, uh, different, you know, isomers of cyclohexane, uh, and things like this, we'll specifically look at the conformations of the chair and we will know that it will turn out that there will be two conformations of chairs in balance with each other and there will be such small amounts of spinning boats or boats that we just ignore them completely, so you have to be aware of the names and you might get an option question multiple.

Somewhere along the way it says which of the following are all cyclohexane confirmations boats twisted boats chairs made very well life is good so now let's talk about these chair conformations and we call them chair conformations you can see if we put a my big head there and then we'll put my big feet there and then we'll put my arms and I'll be reading a book here and like this here is the head of the chair here is the foot of the chair like this and then these two right parallel lines here and here would be the arms of the chair and that's where it gets its name from so it's like sitting in a reclining chair so now it turns out they're chairs like one of those old school lounge chairs that you might have gone out by the pool and you can take your foot of the chair and turn it up to where it is now in this position and you can turn your head and turn it all the way down to where it is now in this position and just Imagine that you were still lying in the chair after you had turned this side down and this side up, so now my head would be down here and I would feel very uncomfortable with a broken back, but you can see that now when chair converted to its other chair conformation, my head would now be at the foot of this chair and then my Your feet would be up on the new head of the chair and it would be nice to go here, but the important thing here is that when we do it, we call this a chair turn, so where your head normally is becomes the new foot of the chair. chair and where the foot used to be becomes the new head of the chair and then these two arms just change the orientation a little bit, but they're still the two arms of the chair, so let's get me out of that picture and I'm glad it helped you. liked my artwork, it's terrible.

I draw confirmations of cyclohexane chairs much better than anything not related to chemistry, so it turns out. when we draw these cyclohexane chair conformations, they differ a little bit in how we draw them from this guy. Now a lot of times we'll refer to this as the flat top forming and the reason we call it that is because if we do this the board and we turn it to the side, what we're really doing is if we look at it from an aerial view, like this that's what we see, however, if we take it off the board again and hold it to the side if we look from the side that's what we would actually see is these chair conformations now in this beautiful flat top conformation each carbon has two bonds additional additions to the bonds in the ring and in pure cyclohexane those would be two hydrogen atoms and one is a wedge and the other a dash on each carbon, so I'll draw just those two, but again, each carbon has a wedge and a hydrogen discontinuous, but in the conformations of the chairs we do not draw wedges or dashes, so everything will be represented. by a straight line and there's a certain perspective that we inherently expect you to understand, so this is one of those places where I highly recommend that you build a model and we'll take a look at a model here in a moment.

However, there are two types of positions that we define in these saddle conformations and we call them axial and equatorial now. If you look at your axial skeleton, your axial skeleton includes your skull and your spine and maybe a little bit of your ribcage, which is your actual position. The skeleton runs vertically up and the same thing here, your axial positions here will be perfectly vertical positions, so if we take a look and I'll put them in this one over here on the right if you start with either of their top two positions here and one thing to keep in mind if you are drawing your cyclohexane chairs here keep in mind that they are made of three parallel lines their arms are pillow lines these are parallel lines and these are parallel lines and it kind of matches what we see here so here we have these two parallel lines, these two are parallel lines and then these two are parallel lines, three sets of parallel lines that are mirrored here as well, but we're looking at it from a slightly different perspective, so again. the arms are parallel so these two are parallel and then these two are also supposed to be parallel lines so that's how you draw your chairs with three sets of parallel lines here and then if you go to any of your highest points here and draw a link up, that's your axis, so again, you could have started it here, you could have started here, you could also have gone to your low points and drawn links down, but the way it works is that then alternate up down around the ring every two carbons so this one goes up this one is going to be down up down down up down cool and that's all the axial positions here okay then and then each carbon is going to have one of these axes and notice that they alternate one point up one point down one point up one point down one point up one point down now I have the equatorial and students generally don't struggle with the axes as much as they do with the equatorials, so let's see how this works, but again, if we took the confirmation of this aerial planer and turned it sideways, the wedges would point up and what's pointing up here here and here corresponds. to the wedges now, the truth is that we could, instead of flipping this up, we could take this and flip it down and that wouldn't necessarily be true, but I'm going to turn it sideways like this every time so that the wedges always correspond. to something on my chairs that points up, FYI, cool, so this would be a wedge, a wedge and a wedge and then these three axes would all be dashes.

Now, one thing to keep in mind: if your axial position is onewedge, then the equatorial will have to correspond to the other bond in that carbon a dash, but if its axial corresponds to the dash, then the equatorial will have to correspond to the wedge, so again in this structure each carbon has a wedge and a dash , but in this structure each carbon has an axial or an equatorial and the unfortunate thing is that half of the axials that point upwards correspond to wedges, half of the axials that point downwards correspond to dashes and the same will happen with the equatorials.

Half of the equatorials that will point down correspond to dashes and half of the equatorials that point up correspond to wedges and the way you remember this is that if your axial points up to be a wedge, then your equatorial will have to point up. down and be a dash, the problem is that the equatorials don't point directly down like the axials your axial skeleton again runs vertically towards you and that's why they all point up and down, that's why they called them axials, but your equatorials, if you really took a look inside your head, they would be going around the equator of this molecule and pointing. outwards and then one will point and up and the next one will point and down, but they will not point up or down, they will be inclined up or down, but also outwards, etc.

The way you can remember how to draw them is to cut your molecule in half, your chair conformation molecule in half, so that all the equatorials on the right side of these three carbons are tilted to the right and all the ones on the left . The left side will be tilted to the left and then you'll be able to tell if they're tilted up or down just because it's going to be opposite to whatever the axis is, so in this case, on the left side of the molecule, these three all go to have an equatorial inclination to the left and in this case it will be towards the left and down because the axis points up in this one it will be towards the left and up because the axis points down and in this one it would be towards the left and down because the axial points up, great, so it's not that bad, so these three will all be tilted to the right and again in the opposite direction to the axial, so once again here this axial points down, so this It would be up and to the right this axial. is up, so this is down to the right, this axial down, up and to the right, cool and you just need to get really good at drawing these now.

One thing to keep in mind are these two here that students get wrong more often than anything else. They're like, oh, I should put it back in the middle of this big empty space. No you should not. It leans left and down on this one, up and right on this one, not right in the middle of empty space. Great, good way to lose some points in that case. Now it turns out that when you do the chair turn, it turns out that all the links that are axial in this structure will end up being equatorial here and all these that were. equatorial in this structure when the chair is turned over it becomes axial and all of these that are red that are equatorial in this structure are going to be axial and again they will start at any high point and go straight up, then down, then up, then down, then up, then down. and we can do the same with these equatorials in this one again they were axial and blue here they are going to be equatorial and blue here and in this case again those on the left side are going to point left and those on the right side a small point to the right like this that this one again with the axial pointing up will be left and down this left and up and this left and up this right and down right and up right and down and now we have all of our equatorials drawn to this one as well.

Now it turns out that they don't have the same instability when you put an axial or equatorial substituent, it turns out that if you have a choice, substituents prefer to be equatorial than axial, so if I had a substituent right here, where this blue is here and it was axial in the moment when the chair turns over, it would correspond to this one here, now it would be equatorial and what we discovered I would rather be in this chair than in this chair and, although you put these two chairs together in equilibrium in the solution, it is not always 50 50.

And It turns out that the larger the substituent, the more they want to be in this chair than in this chair. be equatorial and so the idea is that you can see a couple of different things, so if we take a look at these three axes here here and here that point up, they all point up and they're actually not that far apart in space. They're not that far from each other and the bigger groups you put in here, as you know, with hydrogens it's not that bad, but you start making these groups bigger and bigger and they're going to start colliding with each other more and so on. because it's on all the other carbons and the same thing happens with the three down here, so the axial position of all the other carbons has the possibility of having some steric hindrance and it turns out that it's more than just a steric hindrance, too There is a It turns out that there is also little torsional stress associated with some of the clumsy interactions that occurred for these axial substituents, but as a result we call them three diaxial interactions.

I'm running out of space here, so a three diaxial interactions. And this is why we usually explain why it is better to be equatorial than axial when your axial has these three diaxial interactions, they call them one three just to show that they are on every two carbons that they interact with. each other is great when you are in an equatorial position, there is nothing analogous to this that you experience, in fact when you are in an equatorial position you also don't have any clumsy interactions to worry about, so one thing that is good about these chairs is that these are in staggered conformations each carbon carbon bonds you look around you look down everywhere in the structure you will see that it is a stagger in fact, we will see the model here in a moment you will see it like this, but they are all It will be in staggered conformations, but clumsy interactions are possible, but only for an axial substituent, at least as long as it is the only substituent, it turns out that if you have two adjacent atoms that you know in the ring, they both have substituents.

Well then maybe an equatorial could have a clumsy interaction, but if you only have one substituent, it will have clumsy interactions when it's axial, it won't have any when it's equatorial, so another reason it's better to be equatorial, but usually those clumsy interactions and those things we just include and boil it all down to this idea of ​​one of three diaxial interactions, so again we have the steric hindrance between the groups on every two carbons in the axial position and then the associated clumsily phantom interactions that go along with it and that's why that again it's better to be equatorial than axial and from here I recommend that you build the model, so I'm going to take and let you see a model and again, unfortunately, my model kit is really small, so let's take a look at it peek under my document camera, so let's take a closer look at the structure of cyclohexane here.

If you look at it from above, here we could call it the flat top view. Cyclohexane looks like you can You see the hexagon like this, but you don't realize that it is not a planar structure until you look at it from a side perspective and it is this side perspective that we look at when drawing the chair conformations, so we have the head of the chair here. and the foot of the chair here and then the two arms of the chair here now, if we turn it one carbon we can adjust who we call head and foot and in this case now we call this guy head and this foot and these would be the arms, so there's a little bit of arbitrariness, but you'll see that we have three sets of parallel lines, we have parallel lines here, we have parallel lines here and then we have parallel lines here. and that's all part of drawing the conformations of your chair.

Now it turns out that we also have two different types of positions. I have them diagrammed here in pink and green and the green one here is what we call axial positions, they point up and up. down just like your axial skeleton, your skull and your spine, so in this case we have the equatorials that go around the molecule around the equator, so to speak, again, if you look at it from the top view, you can see they go around the equator so all the others tilt down and then up, so the equatorials here don't experience any significant amount of steric strain, but it turns out that the axial ones, if you look at them, these three all point up and they are parallel to each other and therefore there is some torsional stress as well as steric stress associated with that so the ones at the bottom do the same thing at the bottom here are all these three points down in green uh experiencing the same thing and since it's every two carbons, we refer to them as one of three diaxial interactions, so if you also look down one of the bonds here and we're going to look at this bond right here, you'll see that in cyclohexane we're in a perfect situation. staggered arrangement and which also leads to the stability of the chair's conformation.

You'll also notice that with the axial position you have a clumsy interaction with this carbon right here, so there is an incorrect 60 degree separation, but for the equatorial positions, in pink here again, there is no error with the carbons in the ring, they are pointing exactly opposite to the carbons in the ring, which is another part of those three diaxial interactions, sometimes people mention that there is a bug there and there are actually a couple of them for each, depending on which bond you look down on. , we can look at this bonus as well and see that this guy has another fumble with this carbon here as well, but whether we look at them or not is not the biggest concern in the world. people just swallow them and call them one of the three diaxial interactions, but it is for this reason that it is more stable for a substituent to be in an equatorial position compared to an axial one, so let's look at our chair conformation again from the beginning. side view, so in this case we have our head and our foot and if you want to do the internal conversion of the chair, then take your foot and lift it up and if all we do is just flip it up, we would have our boat and there would be our substituents of the flagpole, so then take what used to be on the head to your left and swing it down keeping those arms parallel, not the easiest thing in the world in this case, I have a new chair, a new head, a new foot. but if you notice the difference in the positions, now it's all the pink ones pointing up and down, they're the new axes and the green ones are now all equatorial, circling the equator, so to speak, again if we flip it over backward.

The foot goes back up, so I flip the foot back down and notice that all the links turn just a little so that they are arranged in such a way that now again all the greens are axial again and all the spikes are coming back. to be equatorial, this is the conformation of your chair, you should build a model of this at least once, you should do the internal conversions of the chair and see how everything axial flips to everything equatorial. One thing to keep in mind whenever everything axial flips to everything equatorial, let's focus. on this one here so this guy here that's axial now in green when I turn it to the other chair so on the other chair it's now equatorial here but it's still pointing up now it's not straight up it's still tilted up but it still stands out and that's important, as we'll see when we start talking about cis and trans and things like that in a moment, now that we've had a chance to look at the structure a little more carefully and see what happens in those conversions of chair-chair.

I just want to emphasize again how the size of a substituent will affect how much it gives preference to the equatorial position over the axial position and these are the same three that are on the next page. from your study guide here and we see that when it has a chlorine substituent, it strongly prefers the equatorial position, so at equilibrium here 70 percent of the molecules in the equatorial 30 percent will be axial, but with a methyl group. Furthermore, and this is not completely intuitive, you might have a conversation about a methyl group that is larger than chlorine, well, it turns out that it is a carbon with three hydrogens attached to it, and they are also very hard atoms, so so to speak. chlorine has a little more polarization and is softer if it is an electron cloud, but it turns out that it has an even greater preference for being equatorial, so 95 uh of methylcyclohexane molecules will make methyl equatorial only a five percent axial, so you should You know it's more important to make sure that a methyl group becomes equatorial than, for example, a chlorine, so in this case, or even a bromine, it turns out that now it's a group t-butyl and we see that the two groupsbutyl they feel a little bigger. than a simple methyl group and for simply old carbon chains, it would be more important to obtain an equatorial ethyl group than a methyl group or, in this case, we see with the t-butyl group, although 99.99 of the molecules in equilibrium are It will be in the equatorial, only 0.01 percent will actually be axial, so the important thing here is that, again, the larger the substituent, the more preference it has for being equatorial, so we'll start to look at, you know, poly . substituted cyclohexanes and we will often know that the question that will be asked is to draw the most stable conformation.

Well, the most stable confirmation will usually be on the idea that equatorial is preferred to axial, but it doesn't always work out. To make all the substituents you know equatorial depends on how they relate to each other in the structure, but your goal will then be to get them to know twice, get as many substitutions as equatorial as possible, and get the largest substituents. equatorial and it's usually more about getting the largest equatorials first and then as many equatorials as possible after that to get the lowest energy confirmation, but as we saw with the Newman projections, one of the most common questions will simply be draw the most stable conformation.

Alright, one of the other important things when drawing cyclohexane chair conformations is being able to recognize cis and trans now cis and trans are possible in a ring and what we mean by cis is when you have two substituents on the ring that are on the same side now in the top brush conformation, that means both wedges or both dashes, while trans means one a wedge and one is a dash, so here we can recognize the top planar confirmation that they are both wedges, these methyl groups, so they would be cis to each other, but again, because we don't draw wedges and dashes in the chair conformations, we have to Okay, so how do we recognize this in trans?

Well, if you remember, we think of these wedge ties as being analogous to groups pointing up, and so it turns out that cis and trans end up being an up and down thing if two groups point. both up that's cis if they both point down that's cis but if one is up and the other is down that's trans and that's why we don't want to confuse this with being axial and equatorial because it's not about up and down so if you notice here this group points up and this one points up it is not up because it is equatorial but it does point up and because they both point up that is cis so notice the axial of one the equatorial but that has nothing to do with cis trans absolutely so yes You will notice that in this case they both point upwards and this is also cis but in this case both are axial and again in this case one axial one equatorial and it was cis in this case both are axial and that one was cis in this case they're on adjacent carbons, in this case they're one more apart and that's really the difference, but again, the important thing is that cis and trans always means you know both up or both down for cis and one up and one down for trans , so I look at these here and I can see that this axial position here is definitely down and this one is not up or down, so it's definitely not axial, it's equatorial, but it's tilted more down than up, so which are both down.

That's also another way to represent cis, so same thing on the next one here, they're both equatorial, which isn't important for recognizing cis and trans, but they're both slanted down. They are both equatorial but they are both tilted down that is what makes them cis so again sometimes cis will mean both axial or both equatorial sometimes it will mean one axial and one equatorial but the key is it will always mean both up or both down to be cis now one of the other shapes we can take A look at the chair conformation of cyclohexane is with a Newman projection and I like to call it a double Newman projection.

It turns out that you can look down like both arms of the chair at the same time, so if we look down, you know down. Let's do this in red, let's say we look down at this chair arm here and this chair arm here at the same time, what we would discover is that the two front carbons here are this guy and this guy, so would join down by a ch2 which would be this ch2 right here, then you have the back carbons that we would be looking down, just behind those two front carbons that would represent the circles in the Newman projections, and those would be accompanied by a CH2 as well and that's this guy up here, so that's the perspective we're looking at and I chose the two arms because it's easier to see, but the truth is you just turn and look. change of perspective you can look down any two parallel carbon bonds in the ring at once and get a sort of double Newman projection now let's say we want to look at this guy right here well I can see that the front carbon if Rotate this on the side, the front carbon on the left, so it has a methyl group facing up that would be right here, so we don't really want to do that.

I'm going to take it back, guys, so if I want to look. one I definitely don't want to see this one here, let's see which one would be convenient to look at. Actually, I haven't drawn. Ah, there we go, we can see this one here. It will be much better and you. We'll see why in just a second, so we'll look at this link here and this link here and here are our two front carbons and here are our two back carbons and the important thing here is that you want to make sure that your substituents are not on the foot of the chair or at the head of the chair or else you won't be able to see it from this perspective, so I want to choose one of these when looking down, where my substitute is in this case. was in one of the illustrations, the front of the arm on the left arm and the back of the right arm where the substituents were, so I'll be able to see them both on my double Newman quite easily, so in this case of the carbon on the left side, I'm going to draw the relevant hydrogen here and the relevant hydrogens here and here and then on this one we have a hydrogen here and then a hydrogen here and here and so on, if Look at the front carbon on the left side, we see that we have a hydrogen pointing up but then tilted down, that's where the methyl group is and note that it's pointing down, but the back carbon on the left is this one. guy here, so you have two hydrogens, one pointing down, one pointing down, and then one leaning up, there, great, now the right side is going to be again looking at this, this is this arm that would be on the right side and the front carbon, again this guy has a hydrogen pointing up and then one tilted down right there and then the back carbon has a hydrogen tilted up to the right, but then down has our substituent a methyl group on this cool case. and the key is to realize that this methyl group is tilted down, so it's not axial, it's equatorial, but this one here is straight down, so it's axial, not equatorial, but it still points down, so one is equatorial, one is axial and that doesn't matter that far away.

As far as the trans sister goes, but they both point towards another way we should recognize that a molecule is cis, so with these double Newmans you have to realize that the positions that point up and down on your arms here they will be axial and then the ones that are tilted up or down, are your equatorials, so again, the axials on this side, the equatorials on this side, are just another perspective to look at the conformations of your cyclohexane chair, like this which we will now look at several different varieties of trans isomers here, in this case, at the top.

On the planer, I see a wedge, a dash, and again, that corresponds to one that points up when you look at it from the side and one that points down and that's definitely what trans is, and again, if we look at this in the saddle, it is not about axial or equatorial again. it's about how they are tilted up or down and this one is tilted up and this one is tilted down and that is actually trans, they are both equatorial but again the key is one up and one down so here we have an equi, sorry, an axial that points down here we have an equatorial that points up the key is some up some down that is trans the same thing here they are both equatorial but this guy is up this guy is down that is trans and finally this guy here both they're axial but this guy's up, this guy's pointing down and that's trans and the same thing here in our double Newman projection, we can do the same kind of thing, so in this case I'm going to take a look one more time on the arm of this chair, both of us. arms of this chair, so we have our two front carbons, now we have our two back carbons and it is one where our substituents are both on the arms of the chair, so it will be easier to see in this case in the In the left arm I can see that I have hydrogen in the axial axis in the front carbon and a hydrogen in the equatorial in the front carbon, so in this case a hydrogen in the axial and a hydrogen in the equatorial in the front carbon and then I can see on the back carbon I have on the equatorial that is where the methyl group is and then I have a hydrogen on the axis pointing down and again on the back carbon the hydrogen is on the axis pointing down and then I have the methyl group on the equatorial position on that back carbon and then on the right arm I can see that I have a hydrogen on the axis pointing up and then the methyl groups on the equatorial, so the hydrogen and the axial pointing up, but the methyl group it's on the equatorial right here and then on that back the carbon has an axial hydrogen pointing down and an equatorial hydrogen tilted up to the right and that again points down and tilted up and to the right and So now We have everything focused on this double number.

What I can see from here is that this methyl group is definitely equatorial, it's not straight up or down, it's tilted, but it's definitely tilted up is the key and this one is also equatorial and it's definitely tilted down and, so as we identified these one here, one up and one down, they're both equatorial, so the key is one up and one down, that's what makes it trans, so finally now we're going to draw some chair conformations of cyclohexane. and again, the most common question you'll get is that they'll give you a cyclohexane, either by name or in the top planer confirmation, like I did here, and then ask you to draw the most. stable confirmation, well, that means you know you're going to draw a chair conformation.

Sometimes they will ask you to draw both chairs and circle the most stable one or sometimes they will just ask you to draw only the most stable one, so in my exercise here. We're just going to try to draw the most stable one, so I've got the top planers and a chair on the board, and now we're going to do our best to add the substituents in the right places. in this case, they are both methyl groups and they are in a ratio of 1 4 if you look, if I define either of these as in position one and then the other numbering around the string here, whether it goes clockwise clockwise or counterclockwise, it's going to be at position four, same thing on this one, if I do this one at the same position one, then I want to turn clockwise because it would come to this type at a lower number, so this would be called a 1 3 ratio in cyclohexane.

We'll see that that's going to have an impact here now, they're both cis isomers, we can confirm that when we're done here, but in this case, because they're both methyl groups, usually when I'm trying to draw the lowest energy conformation, my goal is to first make that the largest is equatorial, they both have the same size as the substituents, they are identical, so my goal is to make one of them equatorial, if possible, it will always be possible to get the equatorial. so in this case it's a wedge and a wedge means it points up, keep that in mind and if you look at the different equatorial positions that point up, one of them is here, this one would tilt down, this one would tilt up and this one go down and then this one tilts up and then this one would go down and those are the three equatorial positions that point up and I'm just going to choose one of them to be this methyl and it doesn't really matter which one is totally my choice so I'm going to choose this one and I've tilted it up a little bit more than I wanted, so let's stretch it out a little bit, but I'm going to leave it there, great.

So, that will be my number one. Let's say it's my number one. I was putting number four, but let's make it cool, that's number one. So if that's number one, then again, in this case I never turn clockwise to get number four. that's one, two, three andfour, great, obviously, we don't use this link here and we don't use this one here now on this carbon, there are two links here, one that points up on the axis and one that is tilted. down and left and that is equatorial and again I can't choose if it is actually equatorial I just have to make sure that it is also a wedge and points up and in this carbon the link that points up is the axial one, not the equatorial and that is where this other methyl is going to go and in this case, I have to have an axial and an equatorial.

If we do the chair spin right here, it would end up being equatorial. It wasn't axial because they always swap places like actually let's draw it for fun so this charcoal right here so this would be one two three and four so one and two is still the arm of the chair was still the arm here three was the foot of the chair, now it's become the head of the chair, and then four is right next to it, so in this case this is up and equatorial on carbon one, and now it's going to be an axis up on carbon one and a lot of times we would draw this, let's make it look a little better and give ourselves some space by placing it over the front of that link, so there's that one here.

I could draw it really small right there if I wanted to, but I'll have it. going right in front, I guess I should actually make that solid line and hit this one, there we go, so go in front of that other carbon-carbon bond right there, so there's one of them and that's on carbon one and then in carbon. four, it points up here, it will still point up, but here it was axial, here it will be up and equatorial and the up position on carbon 4 here is that equatorial, there's our other methyl group, great, so notice that the part up down no Don't change this points up this points up they were cis both still point up but here this equatorial era is now axial this axial era is now equatorial and these would have the same stability, it wouldn't matter which of these we drew because with one axial, one equatorial and then being identical, these would have exactly the same energy and that would be a 50 50 representation in an equilibrium solution, let's take a look at the next one here so here instead of a relationship of 1 4 it is a relationship of 1 3 and again I want to choose one of these that is at least equatorial.

My choice now we need to keep one thing in mind, so I'm going to choose the position. 1 here to be equatorial, we already know what it points to. I'm going to make sure I'm back in one of those equatorials. Now the truth is that I don't have to put this out before predicting whether it will go or not. become equatorial also because if you remember, all the equatorials alternate just like the axial ones, the axes go up, down, up and the equatorials do exactly the opposite, down, up, down, up and, in this case, if up It is equatorial.

I'm going to make sure that half of the equatorials point up, so if he's going to be equatorial and up, then his equatorial is going to point down and his equatorial is going to point up and his is going to point down and his equatorial points up and its points down. Let's go back here and notice that we just predicted where that methyl group is going to point, so this one again up is equatorial, down is equatorial, up is equatorial, which means the methyl group over there is also going to be equatorial, so we can predict. without even drawing the chair necessarily, let's figure it out so carbon 1 and again I want to make sure that I can choose one of the equatorials that points up to be it and again it's my choice now I chose this to be carbon one on this last one, I I'll make sure it's this one this time, this one will be carbon, it's totally arbitrary, I just want to pick one of these three because these are the equatorial ones that point up, the other three equatorial ones point down, cool. so I'm going to choose it to be the methyl group here again.

I could have chosen any of those three. So if I numbered clockwise here, I should number clockwise here with a ratio of one to four, it didn't really matter. one way or another because it's equidistant, but with a ratio of 1 3 or 1 2 you should really number it exactly the same way if I go clockwise from one to three it should go clockwise of the clock from one to three here and in this case that The position up is what I need in position three, that is the other equatorial and in this case it is up and equatorial and this one is also equatorial up, so if I asked you which Of these two structures it has the lowest energy conformation, this does not matter. how you cut it you will get one axial and one equatorial but for this one in this commit both are equatorial now in the other chair commit which again they didn't ask us to draw but let's put it there so in this case position one was the foot of the chair, now it is the head of the chair, but now that equatorial that was pointing up will now be an axial that points up.

Same thing here on carbon 3 here used to be an equatorial pointing up. Now it's going to be an axial pointing up and so in this case they're both axial, so the one on the left here is much more stable, if I were to say which of these two different cyclohexanes has a more stable effect. conformation will be this because I have a chance for both methyl groups to be equatorial, so again, you might see this appear by simply drawing the lower energy conformation or I could draw both and circle the lower energy conformation. Those are the two most common ways a question like this is asked.

Well, we have two more examples to solve here and again, this one has two identical methyls, so I'll choose one of them to make sure it works. it's going to be equatorial and I'm going to make sure it's this guy here, it's going to be an equatorial that's pointing down and then we can again predict whether it's going to become equatorial or not, in this case if the equatorial on this carbon is pointing down, then the equatorial on this carbon will point up and the equatorial on this carbon will point down, which means that this wedge will not become equatorial, so if I make it equatorial it will get stuck being axial, but then the opposite saddle would be flip-flop, like this that no matter what I do here, both chairs will have an axial and an equatorial again, they will be equal in energy, whichever of the chairs would be the one with the lower energy, so in this case, if I do, in this case I want an equatorial that points down, so the equatorials that point down are this one and then they're all the other carbons, as long as I do any of those, this methyl group here, I'm set and again I could have chosen either one. they're the same size, it doesn't really matter in this case, but I'm going to choose it like this and if I choose it to be number one, then I should number it clockwise until I get to position three to make it one. so he will be in position three and in this case in position three I just have to choose what is on top, well the equatorial points down, I can't use it, I have to use what is on top, which in this case happens to be axial and that's where that other methyl group will be and again, in this case, in the chair change, it will become equatorial and it will be axial, but either way I get an axial methyl and an equatorial methyl, either one would have the same energy and the lowest energy, therefore, a conformation.

Now the first really fun example we get is a trisubstituted cyclohexane and again you have a double target. If you are trying to get the lowest energy confirmation, you want to get the largest equatorial group and as many equatorial groups as possible. and in this case, the t-butyl group right here you should recognize that, as a tert-butyl, is what you really need to worry about. They are both just little methyls and if you remember the methyl groups you know they preferred equatorials with a similar tune. 95 to 5 but the t-butyl group preferred equatorial like 99.99 to 0.01 percent, we really have to worry about its size, so its larger size than the methyls means it really really wants to be equatorial, so really we have to worry about it, so if I want the lowest energy confirmation, it's definitely going to be about getting it to be equatorial, so I'm going to call it position one and it's definitely pointing up and this guy is going to point up and this guy is going to point. down now I'm going to make sure that this guy becomes equatorial in that lower energy conformation so that we can predict once again where these guys will actually be, they will be axial or equatorial, so if in this case the equatorial points up so in this carbon the equatorial would point down here it would point up it would point down not up so this will have to be axial so once again the ax or the equatorial points up here points down here points up here points down here points up it's not going to become equatorial as well so it turns out we're a bit stuck the best confirmation we're going to get is that it will have that equatorial tbil group for sure but unfortunately both methyl groups will end up being axial instead , let's draw it right, so in this case I can choose where the carbon 1 is.

I just want to make sure that I again choose a position where the equatorial is pointing up and it's there, it's there or it's there and again it's totally my choice and I'm just going to choose this position right here, so that's where I'm going to put that t-butyl group so that he's there and then if it's number one, I'm going to number around counterclockwise two three four so counterclockwise two three and four and in this case in position three I need to choose the bond that points down, well the equatorial one, put it up is the axial one that will point down there is a methyl group there and in position four i need the link pointing up which is another axial the equatorial one points down so again we are stuck with both methyl groups being axial but the t-butyl group is equatorial but this will have much less energy than if these two were to be equatorial. and it would have to go back to being axial with those t-butyl group fans or butts, if you have a t based on your cyclohexane, make sure it goes back to being equatorial if you're looking for that lower energy conformation now if you found this lesson helpful.

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