cycle alkanes that'll be the topic of this lesson in my organic chemistry playlist and more specifically we're talking about cyclohexane more than any of the rest of the cycle alkanes and then even on top of that specifically about the chair conformations of cyclohexane as well now this comes on the heels of an entire chapter on alkanes we spent the first half of the chapter going through 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 lesson will be talking about the confirmations of cycloalkanes and again most importantly the chair confirmations of cyclohexane 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 both understandable and even enjoyable now this is my brand new organic chemistry playlist i'll be releasing these lessons weekly throughout the 2020-21 school year so if you don't want to miss one subscribe to the channel click the bell notifications you'll be notified every time i post a new video so cycloalkanes and here we've got cyclopropane cyclobutane cyclopentane cyclohexane and cycloheptane on the board and want to first 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 on here so we got one here and here and here and here and as a reminder the wedged bonds here so i mean that these hydrogens are coming out of the board the dash bonds mean the hydrogen's going into the board so the big thing i want to look at is the bond angles within the triangle here within that equilateral triangle and so for nickel out of triangle those bond angles are 60 degrees and that is a problem so the carbons are all here sp3 hybridized and being sp3 hybridized they want to adopt bond angles of 109.5 well 60 degrees is not even close so and it turns out when your angle is much lower than what it wants to be that's associated with what we call angle strain cool and cyclopropane has a ton of angle strain now if you looked at cyclobutane cyclobutane is pretty close to a square it turns out it kinks so it's not perfectly planar it's not perfectly two-dimensional and so these angles aren't exactly 90 degrees they're like 88 degrees but close enough so in this case it has a little less of this angle strain associated with it now angle strain is just a part it's one of three components of some larger topic we call ring strain and there's two other parts to this ring strain and there are two things that you've already visited before one is steric strain atoms bumping into each other and the other is torsional strain repulsion between the electrons in bonds and as you recall we studied this in the last lesson on newman projections we learned that eclipse conformations have significantly more steric and torsional strains then your staggered conformations one of the reasons they're higher energy well it turns out that in cyclopropane here so if you look down this carbon-carbon bond here you'd find out that these two hydrogens line up perfectly and these two hydrogens line up perfectly and it is an eclipsed conformation and therefore cyclopropane not only has a fair amount of angle strain it has a fair amount of both steric and torsional strain and the combination of all three of these we'd say that cyclopropane has a fair amount of ring strain in fact it has more ring strain than any of the other cycloalkanes so and a big part of that does come down to that angle strain now if we looked at cyclobutane that angle is still not 109.5 but it is far closer than this guy so it turns out cyclopropane actually even has bent bonds when the orbitals overlap to form those carbon carbon instead of being dead on so they're angled a little bit and we say they have bent bonds and it turns out this these bonds are actually much weaker than in the other structures and cyclopropane will do certain chemical reactions that under this other cycle alkanes will do and stuff so it's so unstable uh that that like i said it's got some chemical reactivity associated with it that the others don't all right so it turns out by the time you get all the way over to cyclohexane here cyclohexane has the ability in its chair conformations to have zero ring strain so but it turns out that as you get smaller and smaller and smaller you're gonna have more and more and more of this ring strain associated with it but it turns out once you go bigger than six carbons as well so now the sudden you can't get the perfect bond angles you can't get 109.5 degree angles exactly like you could in the chair conformations of cyclohexane and so as you get bigger than six carbons in your ring you also start encountering ring strain as well now we won't look at bigger too often so but we will look at this trend that the smaller the ring the greater the amount of ring strain and we get a little technical here we say there's more ring strain per carbon atom is usually how we phrase it so cyclopropane has the greatest amount of ring strain per carbon atom in the ring all right so because cyclohexane is the most stable by far so it therefore is the most common ring by far it turns out nature and things of a story and it's the one we're gonna study the most with the rest of this lesson now it turns out cyclopropane is actually fairly common as well in nature five-membered rings so it but it does have a little bit of ring strain but once you get to cyclobutane you have quite a bit of ring strain and then cycle propane a ton of ring strain associated with it all right so now we're going to spend the entire rush of the lesson talking specifically about cyclohexane now it turns out the cyclohexane can actually adopt a few different conformations and the chair conformation is going to be the most important one and the one we're going to spend pretty much the rest of the lesson studying but i do just want to briefly mention the other confirmations and the other one is a boat but it's higher energy than either of these chair conformations and this boat confirmation has a couple of hydrants we often like to identify called flagpole hydrants and if you start replacing these flagpole hydrogens with bigger groups they're going to hysterically hinder each other even more and more and stuff like this but it's one of the reasons that is this is a higher energy confirmation than these chair conformations now it turns out there's also things called twist boats where this just kind of twists some of the angles a little bit and stuff like that and only thing you really got to know is that cyclohexane has some other confirmations besides the ones we're going to spend the entire lesson studying that are called boats or twist boats and they're higher energy and therefore there's not a lot of them actually present in solution when we start looking at cyclohexane uh different you know cyclohexane uh isomers and stuff like this we're going to be looking specifically at the chair conformations and knowing that there are gonna turns out to be two chair conformations in equilibrium with each other and there'd be such tiny amounts of boats or twist boats that just we ignore them entirely so you should be aware of the names you might get a multiple choice question somewhere along the way that says which of the following are all confirmations of cyclohexane boats twist boats chairs done great life is good so now let's talk about these chair conformations and we call these chair conformations you can kind of see if we put a my big noggin right there so and then we'll put my big feet right there and then we'll put my arms and i'll be reading a book right here and so here's the head of the chair here's the foot of the chair so and then these two right parallel lines right here and right here would be the arms of the chair and so that's where getting gets its name so it's like sitting in a reclining chair like so now it turns out this is chairs like one of those old school lounge chairs you might have had out by the pool and you can take the foot of the chair and flip it up to where it's now in this position and you can flip the head and flip it all the way down to where it's now in this position and let's just pretend i was still laying on the chair after i'd flipped this side down and this side up so now my head would be down over here so and i would be really uncomfortable with a broken back so but you can see that now in when the chair converted to its other chair conformation my head would be now at the foot of this chair so and then my feet would be up at the new head of the chair and so good to go here but the the big thing here is that when we do this we call it a chair flip so where your head is normally becomes the new foot of the chair and where the foot used to be becomes the new head of the chair and then these two arms just kind of 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 you liked my artwork which is terrible i draw cyclohexane chair confirmations far better than i draw anything not related to chemistry all right so it turns out when we're drawing these cyclohexane chair conformations they differ a little bit in how we draw them from this guy now oftentimes we'll refer to this as the overhead planar conformation and the reason we call it that way is that if we pull this off the board and turn it sideways what we're really doing is if we looked at it kind of from the overhead view so that's what we kind of see so however if we took it again off the board and held it up sideways if we look at it from the side that's what we'd actually see is these chair conformations now on this lovely overhead planar conformation every carbon's got two additional bonds besides the bonds in the ring and on pure cyclohexane those would be two hydrogen atoms and one's a wedge and one's a dash on every single carbon so i'm going to draw just those two in but again every carbon have a wedge and a dashed hydrogen but on the chair conformations we don't draw any wedges or any dashes so everything is going to be represented by a straight line and there is some perspective that we just kind of inherently expect you to understand so this is one of those places where i highly recommend you build a model and we'll take a look at a model here in just a little bit so however there are two types of positions we define on these chair 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 rib cage that's your actual skeleton it runs right up the vertical of you and so same thing here your axial positions here are going to be perfectly vertical positions so if we take a look and i'll put them on this one on the right here if you start with either of your top two positions here and so one thing to note if you're drawing your cyclohexane chairs here note that they're made of three parallel lines your 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've got these two are parallel lines these two are parallel lines and then these two are parallel lines three sets of parallel lines that's reflected 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 supposed to be parallel lines as well so that's kind of how you draw your chairs with three sets of parallel lines here and then if you go to either of your highest points here and draw a bond straight up that's your axial so and again you could have started it here you could have started here you also could have just as well gone to your to your low points and draw drew bonds straight down but way it works is that then they alternate up down all the way around the ring every other carbon so this one's straight up this one's going to be straight down straight up straight down straight up straight down cool and that's all of the axial positions here all right so and then every carbon is going to have one of these axials and notice that they alternate one point's up one point's down one point's up one points down one points up one point's down now i've got the equatorial and the students don't usually struggle as badly with the axles as they do with the equatorials so we're going to kind of look at how this works but again if we take this overhead planer confirmation and turns it up sideways the wedges would point up and so what points up here here and here kind of like corresponds to wedges now the truth is i mean we could have instead of flipping this up we could take this and flip it down and that wouldn't necessarily be true but i'm gonna flip it sideways just like that every time so the wedges always correspond to something on my chairs that point up just an fyi cool so this would be a wedge a wedge and a wedge and then these three axials would all be dashes now one thing to note then if your axial position is a wedge then the equatorial is going to have to correspond to the other bond on that carbon a dash but if your axial corresponds to the dash then the equatorial is going to have to correspond to the wedge so again on this structure every carbon's got a wedge and a dash but on this structure every carbon's got an axial or an equatorial and the unfortunate thing is that half the axials point up correspond to wedges half the axials point down correspond to dashes and the same will be true of the equatorials half the equatorials will point down correspond to dashes and half the equatorial point up and correspond to wedges and the way you remember this then is that if your axial points up to be a wedge then your equatorial is going to have to point down and be a dash the problem is the equatorials don't point straight down like the axials your axial skeleton again runs right up the vertical of you and that's why they all point straight up and straight down that's why they called them axial but your equatorials if you actually took a look at your overhead they would kind of be going around the equator of this molecule and pointing out and so one would point out and up and the next one will point out and down but they're not going to be pointing straight up or down they're going to be slanted up or slanted down but also slanted out as well and so the way you can remember how to draw these is cut your molecule in half your chair conformation molecule in half so all of the equatorials on the right hand side on these three carbons are going to be slanted to the right all of the ones on the left hand side are going to be slanted to the left and then you can know if they're slanted up or down just because it's going to be opposite of whatever the axial is so in this case on the left-hand side of the molecule these three all are going to have equatorial slanted to the left and in this case it'll be left and down because the axial pointed up in this one it'll be left and up because the axial pointed down and on this one it would be left and down because the axial pointed up cool so it's not so bad so these three all going to be slanted to the right and again opposite in direction of the axial so once again here this axial points down so this would be up and to the right this axial is up so this is down into the right this axial down so up and to the right cool and you just need to get really good at drawing these now one thing to note it's these two right here that students mess up more often than anything else they're like oh i should just put it right back out in the middle of this big empty space no you should not it slants left and down on this one up and to the right on this one not right out in the middle of the empty space so cool good way to lose some points on that case now it turns out when you do the chair flip so it turns out that all of the bonds that are axial in this structure are going to end up being equatorial here and all these ones that were equatorial in this structure when the chair flips become axial and so all these ones that are red that are equatorial in this structure are going to be axial and again start at either high point and go straight up then down then up then down then up then down and we can do the same thing with these equatorials on this one again they were axial and blue over here they're going to be equatorial and blue over here and in this case again the ones on the left-hand side are all going to point left and the ones on the right-hand side a little point right so this one again with the axial pointing up will be left and down this one left and up and this one left and up this one right and down right and up right and down and now we've got all of our equatorials drawn in on this one as well cool now it turns out they're not equal instability when you put a substituent axial or equatorial it turns out so if you've got a choice substituents prefer to be equatorial than axial so like if i had a substituent right here in this where this blue one is right here and it was axial by the time it chair flips it would correspond to this one right here it would now be equatorial and what we find out is it would prefer being in this chair than in this chair and so even though we get these two chairs in equilibrium together in the solution it's not always 50 50. and it turns out the bigger the substituent the more they're going to want to be equatorial and so the idea is that you can look at a couple different things so if we take a look at these three axials here here and here that all point up so these all point straight up and they're actually not that far separated in space they're not that far from each other and the bigger you put groups here so as you know with hydrogens it's not so bad but you start making these bigger and bigger groups and they're going to start bumping into each other more and so because it's on every other carbon and we have the same thing going on with the three that are down here as well so every other carbon's axial position has a chance to have some steric hindrance and it turns out it's more than just steric hindrance there's also a little torsional strain in there as well associated with some of the gauche interactions that happened for these axial substituents it turns out as well so but as a result we call these one three diaxial interactions i'm running out of space here so one three diaxial interactions and so this is the reason we usually explain why it's better to be equatorial than axial when your axial you have these one three diaxial interactions they call them one three just to show that they're on every other carbon are the ones that are interacting with each other cool when you're equatorial there's nothing analogous to this that you experience in fact when you're in an equatorial position you don't have any gauche interactions to worry about as well so one thing that's nice about these chairs is that these are in staggered conformations every carbon carbon bonds you look around look down at everywhere in the structure you'll see that a stagger in fact we'll look at the model here in a little bit you'll see that so but they're all going to be in stagger conformations but gauche interactions are possible so but only for an axial substituent at least as long as it's the only substituent it turns out i mean if you've got two adjacent atoms that you know in the ring that both have substituents well then maybe an equatorial could have a gauche interaction so but if like you've just got a single substituent it'll have gauche 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 gauche interactions and stuff we just kind of include that and sum it up all in this idea of one three diaxial interactions so again we've got the steric hindrance between the groups themselves on every other carbon in the axial position and then the gauche associated ghost interactions that go with it and that's why again it's better to be equatorial than axial and from here though i recommend you build the model and so i'm going to take and let you guys look at a model and again unfortunately my model kit is really small and so we're going to take a look at it under my document cam so let's take a closer look at the structure of cyclohexane here if you look at it overhead here we might call this the overhead planar view cyclohexane looks like you can see the hexagon so but you don't realize that it is not a planar structure until you kind of look at it from a sideways perspective and it's this sideways perspective 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 rotate it one carbon over we can adjust who we call the head and the foot and so in this case now we call this guy the head and this the foot and these would be the arms so there's a little arbitrariness but you'll see that we've got three sets of parallel lines we've got parallel lines here we've got parallel lines here and then we've got parallel lines here and that's all part of drawing your chair conformations now it turns out we also have two different types of positions i've got them diagrammed here in pink and green and the green one here are what we call the axial positions they point straight up and straight down just like your axial skeleton your skull and your spine so in this case then we have the equatorial ones that kind of go around the molecule around the equator so to speak so again if you look at it from the top view you can kind of see them going around the equator so and every other one slants down and then up and so the equatorial ones here don't experience any significant amount of steric strain but the axial ones it turns out if you look at them these three all point uh up and are parallel to each other and so there's some torsional strain as well as steric strain associated with that so then the ones on the bottom do the same thing on the bottom here they're all these three point down in green uh experience the same thing and since it's every other carbon we refer to those as one three diaxial interactions so if you also look down one of the bonds here and we're going to look down this bond right here you'll see that in cyclohexane we're at a perfectly staggered arrangement so and that also leads to the stability of the of the chair conformation you'll also notice that with the axial position you have a gauche interaction with this carbon right here so that's a gauche 60 degrees apart but for the equatorial positions so in pink here again there's no gauche with the carbons in the ring they're pointing exactly opposite the carbons in the ring that's another part of those one three diaxial interactions sometimes people mention that there's gauche is there and there's actually a couple of them for each depending on which bond you look down we can look down this bond as well and see that this guy's got another gauche with this carbon up front here as well so but whether or not we look at those or not is not the biggest concern in the world some people just swallow them up and just call them one three diaxial interactions but it is for this reason that it is more stable for a substituent to be in an equatorial position as compared to an axial so let's take a look at our chair conformation again from the side view so in this case we got our head and we got our foot and if you want to do the chair inner conversion so take your foot and flip it up and if all we do is just flip it up we'd have our boat and there'd be our flagpole substituents so then take what used to be at the head on your left and flip it back down keeping those arms parallel not the easiest thing in the world to do in this case i got a new chair new head new foot but if you notice the difference in positions so now it's all the pink ones that point straight up and straight down they're the new axials and the greens are now all equatorial going around the equator so to speak so again if we flip it back flip the foot back up so and i'll flip the foot down and notice all the bonds rotate just a little bit so that it's arranged in such a way that now again all the green ones are back to being axial and all the peak ones go back to being equatorial this is your chair conformation you should totally build a model of this at least once you should totally do the chair chair inner conversions and see how everything axial flips to everything equatorial one thing to note whenever everything axial flips to everything equatorial let's focus on this one right here so this guy right here that's axial right now in green when i flip it to the other chair so in the other chair he's now equatorial right here but he still points up now he's not straight up he's still slanted up but he still points up and that's important as we'll see when we start talking about cis and trans and things of this sort in just a little bit all right so now that we've had a chance to look at the structure a little more carefully and see what happens in those chair chair conversions i i just want to accentuate again how the size of a substituent is going to affect how much it gives preference to the equatorial position over the axial position and this is the same three they're on the next page of your study guide here and we see that when you've got a chlorine substituent so it strongly prefers the equatorial position so much that in the equilibrium here 70 percent of the molecules of the equatorial 30 percent will be axial so but with a methyl group even more so and this is not you know completely intuitive you might have a chat a methyl group is that bigger than chlorine well it turns out so a carbon with three hydrogens attached to it so and they're also very hard atoms if you will where chlorine's got a little more polarizability and squishier if you will an electron cloud so but it turns out that this has a even stronger preference for being equatorial so 95 uh of methylcyclohexane molecules are going to have the methyl be equatorial only five percent axial so you should know that you know it's more important to make sure that a methyl group gets equatorial than say a chlorine so in this case or even a bromine turns out now this is a t butyl group here and we see that the two butyl group notice quite a bit bigger than a plain old methyl group and for just plain old carbon chains like it'd be more important to get an ethyl group equatorial than a methyl group or in this case we see with the t-butyl group though 99.99 of the molecules in equilibrium are going to be in the equatorial only 0.01 percent are actually going to be axial so the big thing here is that again the bigger the substituent the more it's going to have a preference for being equatorial so we're going to start looking at you know poly substituted cyclohexanes and we're going to often you know the question will be asked is draw the most stable conformation well the most stable confirmation is usually going to be about you know this idea of equatorial being preferred over axial but you're not always going to get to make all of the you know substituents be equatorial it depends on how they're related to each other in the structure but your goal then will be to get you know twofold get as many substitutions as equatorial as possible and get the biggest substituents equatorial and it's usually more about getting the biggest ones equatorial first and then as many as possible equatorial after that to get that lowest energy confirmation so but just like we saw with newman projections one of the most common questions will be simply draw the most stable conformation all right one of the other important things in drawing cyclohexane chair conformations is being able to recognize cis and trans now cis and trans is possible on a ring and cis we refer to is when you've got two substituents on the ring that are on the same side now in the overhead planer conformation that means either both wedges or both dashes whereas trans means one's a wedge and one's a dash so so here we can recognize the overhead planar confirmation that these are both wedges these methyl groups so they'd be cis to each other but again because we don't draw wedges and dashes on chair conformations we got to talk about well then how do we recognize this in trans well if you recall we think the these these wedge bonds are analogous to groups that point up and so it turns out cis and trans therefore ends 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's up and one's down that's trans and so we don't want to confuse this with being about axial and equatorial because it's not it's about up and down so if you notice here this group points up and this one points up it's not straight up because it's equatorial but it does point up and so because these both point up that's cis so notice one's axial one's equatorial but that has nothing to do with cis trans in any absolute way so if you notice in this case these both point up and so this is also cis but in this case they're both axial so and again in this case one axial one equatorial and it was cis in this case they're both axial and that was cis in this case they're on adjacent carbons in this case they're one further apart and that's really the difference but again the big thing is that cis and trans always means you know both up or both down for cis and one up one down for trans so looking at these right here so i can see that this axial position right here is definitely down and this one's not straight up or straight down so it's definitely not axial it's equatorial but it is slanted more down than up so they're both down that's also another way of representing cis so same thing on this next one here they're both equatorial which is not important for recognizing cis and trans but they're both slanted down they're both equatorial but they're both slanted down that's what makes them cis so again sometimes cis is going to mean both axial or both equatorial sometimes it's going to mean one axial one equatorial but the key is it will always mean both up or both down to be cis now one of the other ways 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 you can look down like the two arms of the chair at the same time and so if we looked you know right down let's do this in red let's say we looked right down this arm of the chair here and this arm of the chair here at the same time what we'd find out is that the two front carbons here be this guy and this guy so they would be joined together down below by a ch2 that would be this ch2 right here so and then you have the back carbons that we'd be looking down right behind those two front carbons that would represent by the circles in the newman projections so and those would be joined by a ch2 as well and that's this guy up here so that's kind of what the perspective we're looking at and i chose the two arms because it's easiest to see but the truth is as you as you just rotate around and look at change of perspective you can look down any two parallel carbon carbon bonds in the ring at a time and get kind of this double newman projection now let's just say we want to look at this guy right here well i can see that the front carbon if i turn this sideways the front carbon on the left so has a methyl group straight up that would be right here so oh and actually we don't want to do that one i'm going to take that back guys so if i want to look at one i definitely don't want to look at this one right here let's see which one would be convenient to look at actually i haven't drawn ah there we go we can look at this one right here this will be much better and you'll see why in just a second so we're going to look down this bond right here and this bond right here and here's our two front carbons and here's our two back carbons and the big thing here is you want to make sure that your substituents are not on the foot of the chair or the head of the chair or else you're not going to be really able to see him from this perspective so i want to choose one of these in looking down the arms where my substituent in this case was on one of the art the front part of the arm on the left arm and the back part of the right arm where the substituents were so i'll be able to see both of them on my double newman pretty easily all right so in this case on the carbon on the left hand side i'm going to draw in the relevant hydrogen here and the relevant hydrogens here and here and then on this one we've got a hydrogen here and then a hydrogen here and here and so if we look at the front carbon on the left hand side we see that we've got a hydrogen pointing straight up so but then slanted downward that's where that methyl group is so and keep in mind it points down so but the back carbon on the left this guy right here so he's got two hydrogens one pointing straight down one pointing straight down and then one slanted upward right there cool now the right hand side is going to be again looking at this this it's this arm that would be on the right hand side and the front carbon so again this guy right here has a hydrogen pointing straight up and then one slanted downward right there and then the back carbon has a hydrogen slanting up into the right but then straight down it has our substituent a methyl group in this case cool and the key is realizing that this methyl group is slanted downward so it's not axial it's equatorial so but this one over here is straight down so it's axial not equatorial but it still points down so one's equatorial one's axial and that doesn't matter as far as sister trans concerned but they both point down another way we should recognize a molecule is cis and so with these double newmans you should realize that the positions that point straight up and down on your arms here are going to be axial and then the ones that are slanted up or down are your equatorials so again axials on this side equatorials on this side just another perspective in looking at your cyclohexane chair conformations so now we'll take a look at several different varieties of trans isomers here in this case on the overhead planer i see one wedge one dash and once again that corresponds to one pointing up when you look at it sideways and one pointing down and that's definitely what trans is so and again if we look at this on the chair it's not about axial on equatorial again it's about how they slanted up or down and this one's slanted up and this one's slanted down and that indeed is trans they're both equatorial but again the key is ones up ones down so here we have an equi i'm sorry an axial that points down here we have an equatorial that points up the key is ones up ones down that's trans same thing here they're both equatorial but this guy's up this guy's down that is trans and then finally this guy here they're both axial but this guy's up this guy's pointing down and that is trans and same thing here on our double newman projection we can do the same kind of thing and so in this case i'm going to take a look once again down the arm of this chair the two arms of this chair so we've got our two front carbons now we've got our two back carbons and it's one in which our substituents are both on the arms of the chair so it's going to be easiest to see in this case on the left arm i can see that i've got a hydrogen in the axial on the front carbon and a hydrogen in the equatorial on the front carbon so in this case a hydrogen on the axial and a hydrogen on the equatorial on the front carbon and then i can see on the back carbon i've got in the equatorial that's where the methyl group is and then i've got a hydrogen in the axial pointing straight down so and again on the back carbon hydrogen is in the axial pointing straight down and then i've got the methyl group in the equatorial position on that back carbon and then on the right arm i can see that i've got a hydrogen in the axial pointing straight up and then the methyl groups in the equatorial so hydrogen and the axial pointing straight up but the methyl group is in the equatorial right here and then on that back carbon got an axial hydrogen pointing straight down and an equatorial hydrogen slanting up to the right and that's again pointing straight down and slanting up and to the right and so now we've got everything drawn in on this double numen what i can see from here is that this methyl group is definitely equatorial being not straight up or down but slanted but it is definitely slanted up is the key and this one's also equatorial and it's definitely slanted down and so just like we identified these here ones up one's down they're both equatorial so and the key is one up one down that's what makes it trans all right so finally now we're going to draw some cyclohexane chair conformations and again the most common question you're going to get is they're going to give you a cyclohexane either by name or in the overhead planer confirmation like i've done here and then they're going to ask you to draw the most stable confirmation well that means you know you're going to draw a chair conformation sometimes they'll have you draw both chairs and circle the more stable one or sometimes they'll just have you draw just the more stable one so in my exercise here we're just going to try and draw the more stable one so i've got the overhead planers and i've got a chair on the board and we're just going to do our best to add the substituents into the correct places now in this case these are both methyl groups and they're in a 1 4 relationship if you look if i define either one of these as being at position one then the other one numbering around the chain here whether i go clockwise or counterclockwise it's going to be at position four same thing on this one if i make this one at like position one then i want to go around clockwise because i'd get to this guy at a lower number and so this would be called a 1 3 relationship on cyclohexane we'll see that's going to have an impact here now these are both cis isomers we can confirm that when we're done here so but in this case because these are both methyl groups usually when i try to draw the lowest energy conformation my goal is first to get the biggest thing to be equatorial well these are both the same size as substituents they're identical so my goal is then just to make one of them equatorial so if possible so and it'll always be possible to get the one 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 right here this one would then slant down this one slants up and this one would go down and then this one slants up and then this one would go down and so those are the three equatorial positions that all 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 just going to choose this one and i've slanted a little more up than i wanted it to be so let's just stretch that out a little bit but i'm going to stick it right there cool so and that's going to be my number one let's say that's my number one i was putting number four but let's make him number one cool that's number one okay so if that's number one then again in this case i never round clockwise to get the number four that's one two three and four cool now obviously we didn't use this bond right here and we didn't use this one right here now on this carbon right here there are two bonds there's one that points straight up in axial and one that's slanted down and left and that's equatorial and again i don't get to choose whether it's actually equatorial i just have to make sure that it's also a wedge and points up and on this carbon the bond that points up is the axial not the equatorial and that's where this other methyl is going to go and so in this case i got to have one axial one equatorial if we do the chair flip here well then he'd end up being equatorial he didn't be an axial because they always trade places like that in fact let's just draw him out for the fun of it so this carbon right here so this would be one two three and four so one and two is still the arm of the chair it was still the arm here three was the foot of the chair it's now become the head of the chair so and then four is right next to it right there so in this case this is up and equatorial on carbon one so and now it's going to be up an axial on carbon one and oftentimes we would draw this let's make this look a little better and just give ourselves some room by having it kind of come over the front of that bond so there's that one here i could draw it real small right there if i wanted to but i'm gonna have it going right in front of i guess actually i should make that the solid line and give this one there we go so it's passing in front of that other carbon-carbon bond right there so there's one of them and that's on carbon one and then on carbon four it points up here it's still going to point up but here it was axial here it's going to be up and equatorial and the up position on carbon 4 here is that equatorial right there there's our other methyl group cool so notice the up down part doesn't change this pointed up this pointed up they were cis they still both point up but here this one was equatorial it's now axial this one was axial it's now equatorial and these would have equal stability it wouldn't matter which one 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 a solution at equilibrium let's take a look at the next one here so here instead of a 1 4 relationship it's a 1 3 relationship and again i want to choose one of these at the very least to be equatorial my choice now one thing we should note so i'm going to choose position 1 here to be equatorial we already know it points up i'm going to make sure it gets to be in one of those equatorials again now the truth is i don't actually have to draw this out before i predict whether or not he's going to get to be equatorial as well because if you recall the equatorials all alternate just like the axials do the axes go up down up down and the equatorials do exactly the opposite they go down up down up and so in this case if up is is equatorial i'm going to make sure it is half the equatorials do point up so if he's going to be equatorial and up then his equatorial would point down and his equatorial would point up and his would point down and his equatorial points up and his points down back to here and so notice 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 that methyl group right there is going to get to be equatorial as well so we can predict it without even drawing the chair necessarily let's go work that out so carbon 1 and again i want to make sure that i get to choose one of the equatorials that points up so to be him and again it's my choice now i chose this to be carbon one on this last one i'm gonna make sure it's this one this time this is gonna be carbon one it's totally arbitrary i just want to pick one of these three because these are the equatorials that point up the other three equatorials point down so cool so i'm going to choose him to be the methyl group right here again i could have chosen any one of those three so and then if i numbered clockwise here i should number clockwise here with a one four relationship it really didn't matter one way or the other because it's equidistant around but with a 1 3 or 1 2 relationship you really should number it exactly the same way if i'm going clockwise from one to three i should go clockwise from one to three here and in this case that up position is what i need at position three that's that other equatorial and so in this case it's up and equatorial and this one's also up an equatorial and so if i asked you which of these two structures has the lowest energy conformation well this one no matter how you slice it you're going to get one axial one equatorial but for this one in this confirmation they're both equatorial now in the other chair confirmation which again we weren't asked to draw but let's put it in there so in this case position one was the foot of the chair it's now the head of the chair but now that equatorial that pointed up is now going to be an axial that points up same thing here this on carbon 3 here it used to be an equatorial that pointed up it's now going to be an axial that points straight up and so in this case they're both axial and so the one on the left here is more stable by far by far now if i said which of these two different cyclohexanes has a more stable conformation it's going to be this one because i do get a chance for both of the methyl groups to be equatorial so once again you might see this showing up as just just draw the lower energy conformation or it might be draw both and circle the lower energy those are the two most common ways a question like this is asked all right we've got two more examples to work out here and again this one's got two identical methyls so i'm just going to pick one of them to make sure it gets to be equatorial and i'm going to make sure that's this guy right here it's going to be an equatorial that points down and we can then predict again does he get to be equatorial or not so in this case if the equatorial in this carbon points down then the equatorial on this carbon will point up and the equatorial on this carbon will point down which means this wedge that's not going to get to be equatorial so if i make him equatorial he's going to get stuck being axial but then the opposite chair would flip-flop so no matter what i do here both chairs are going to have one axial one equatorial again they'll be equal in energy either chair would be the lower energy one so in this case so if i make so in this case i want an equatorial that points down so the equatorials that point down are this one so and then it's every other carbon so as long as i make any one of those this methyl group here i'm good to go and again i could have chose either one of them they're the same size it didn't really matter in this case but i'm just going to choose him so and if i choose him to be number one then i should number around clockwise till i get to position three so he's one so he's going to be position three and in this case at position three i just have to pick whatever's up well the equatorial points down i don't get to use him i have to use whatever's up which in this case happens to be axial and that's where that other methyl group is going to be and again in this case in the chair flip he's going to get to be equatorial and he's going to be axial but either way i get one methyl axial and methyl equatorial either one would be the equal energy and the lowest energy therefore a conformation now the real fun first example we get of a trisubstituted cyclohexane and again you got a twofold goal if you're trying to draw the lowest energy confirmation you want to get the biggest group equatorial and the most groups possible equatorial and in this case the t-butyl group right here you should recognize that as a tert-butyl he's the thing to really worry about these are both just little methyls and if you recall the methyl groups you know they preferred equatorial to the tune of like 95 to 5 but the t butyl group preferred equatorial like 99.99 to 0.01 percent we really got to worry about him his size so his greater size than the methyls means he really really really wants to be equatorial so we gotta really worry about him so if i want the lowest energy confirmation it's definitely gonna be about getting him to be equatorial so i'm gonna call him on position one and he definitely points up and this guy's going to point up and this guy's going to point down now i'm going to make sure that this guy gets to be equatorial on that lowest energy conformation so and then we can actually again once again predict where these guys are actually going to be out they're going to be axial or equatorial so if in this case the equatorial points up so then on this carbon the equatorial would point down here it would point up here it would point down not up so this is going to have to be axial so once again axe or equatorial points up here it points down here it points up here it points down here it points up that's not going to get to be equatorial as well so it turns out we're stuck a little bit the best confirmation we're going to get is going to have that tbil group equatorial for sure but unfortunately both methyl groups are going to end up being axial instead let's draw it out all right so in this case i get to choose where carbon 1 is i just want to make sure i pick again a position where the equatorial points up and it's either there it's there or it's there and again it is 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 he's there and then if he's number one i'm gonna number around counterclockwise two three four so counterclockwise two three and four and in this case at position three i need to choose the bond that points down well the equatorial put it up it's the axial that's going to point down there's a methyl group there and at position four i need the bond that points up which is another axial the equatorial pointed down and so again we're stuck with both methyl groups being axial but the t butyl group being equatorial but this will be much lower energy than if these two had got to be equatorial and he'd got to be axial again with that t-butyl group noaa fans or butts if you've got a t building upon your cyclohexane make sure he gets to be equatorial again if you're looking for that lower energy conformation now if you found this lesson helpful consider giving me a like and a share pretty much the best thing you can do to support the channel and if you're looking for practice problems or the study guides that go with this lesson check out my premium course on chatsprep.com