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Current time:0:00Total duration:12:29

Video transcript

for all the cyclic molecules we've dealt with so far we've just drawn them as rings for example for cyclohexane cyclohexane we've literally just drawn it as a hexagon so we've drawn cyclohexane like that now we know from the last several videos that all of the bonds for a carbon don't sit in the same plane if we take the example of methane that's the simplest example you have your carbon sitting in the middle you'll have kind of a hydrogen popping out like that another hydrogen that's in the plane of the screen another one that's behind the screen and another one that is straight up so you kind of have this tetrahedral structure and in the case of methane you have that 109.5 degrees on angles carbon likes to form bonds of this shape it won't always be 109.5 degrees it'll be something close to it depending on what the different atoms or molecules are that it is bonded to so given that what would a cyclohexane molecule actually look like if we try to visualize it in three dimensions so to think about that let's think about these two bonds first I'll try my best to draw it in one of its three-dimensional shapes so those bonds right there I will draw like that and then this down here in orange I will draw like this and then this up here in magenta I will draw like that and then let me see in let me see in purple I'll do these two right over here and I'll draw them like this so you have that and like that it so hopefully makes clear that over there is that end over there this end over here is this end over here and this this way that I've drawn the cyclohexane is called a chair configuration chair chair shape and it might be obvious it looks like a chair that's the back of the chair this is where you would sit down on the chair and like I guess the back of your calves would go against here your knees would sit on at some place like that that's called the chair configuration now another configuration that it could be in is called the boat configuration the boat and so if I were to put this exact one in the boat configuration if I take it from a slightly different perspective if I'm looking at it kind of head-on it would look something like this in the boat in the boat configuration make it would look like this it would I want to use the purple it would look like that now the first thing you're probably saying is Sal you said that the reason why it looks like this is because carbon likes to form these kind of tetrahedral or this tripod shaped tripod shaped bonds I don't see the tripod shaped bonds either here or here let me draw that boat a little bit at least this end of the boat a little bit better there you go and let me and you say well I don't see those that tripod shape over there and to see the tripod shape you just have to draw the hydrogen's so let me draw some hydrogen's here so let me draw a hydrogen here that will go that will go straight straight down like this the hydrogen that goes straight down over here a hydrogen that goes straight up over here straight up over here straight down over here straight up over there I've now drawn one hydrogen on every carbon and now let me draw some hydrogen's let me draw a hydrogen here that goes let me draw a hydrogen here that goes straight up over we're not up really to the side over here so a hydrogen there let me draw a hydrogen over here that does the same thing so those guys have their hydrogen's a hydrogen right over here and then let's see this guy needs his hydrogen's still so it'll have a hydrogen that goes down like that and the hydrogen that goes like that and then this guy will have a hydrogen that goes like that and when you see it like this if you look at any any one carbon on this molecule if you look at any one carbon you can see that it's forming the same tetrahedral shape that it has a tripod at every one over here you have that close to roughly 100 910 degree angle between each of the constituents that are bonding to the carbon now I've drawn the different hydrogen's that are coming off of these carbons in different colors and I've done it for a purpose the ones that are going straight up or straight down we call those axial hydrogen's axial axial hydrogen's and the ones I drew an orange that are kind of going to the side in some level we call these equatorial equatorial equatorial these are equatorial hydrogen's and there's reason why it's useful to know that name is when we talk about the different configurations the different chair and boats whether something is equatorial or axial can change if this were to flip up or vice versa and things like that we'll talk more about that in the next video and the reason why they called equatorial is if you think about it and sometimes hard to visualize this bond right here is parallel to this bond right over there and this bond right over here is parallel to this it's parallel the equatorial bonds are parallel to some part of the Ring so that one is parallel to that right over there actually I should even I could even color-coded that this well I don't want to use that same color this is parallel to this and this is parallel to that and we can do it for all the equatorial bonds so for example I don't want to I'm running out of colors here so this right here is parallel to this and this and that over there so we could keep doing it for all of them I could do it one for the other set right here this guy right here is parallel to that guy over there I didn't quite draw it like that but hopefully it makes the idea clear and I'll do one more of these just to show what's parallel to what this bond is parallel to that so the ones that are parallel to some part of the Ring we're calling equatorial and the ones that kind of jump out of the ring that aren't parallel to any other part of the Ring we're calling those axial and the way I've drawn it here the actuals are the ones that point up and point straight up and point straight down we can do the same thing on a boat configuration now one question you might ask is well there's these two configurations both of these would result in tetrahedra we'll type shapes at each of the carbons in fact let me draw it for you so we would have so this axial hydrogen is pointing straight down this one is pointing straight down here these this hydrogen is actually we're going to point straight down because we've flipped it up and then over here you would have a hydrogen points straight up and then one that's kind of pointing down this gives the tripod there to have the tripod over here you'll have to have a hydrogen points a little bit like that one that points a little bit like that one that's pointing a little bit like that along well you can kind of view it along the same plane as this guy would be parallel it's hard to see it in this but he would actually be parallel to that this guy would be out like this and then this guy would have an axial hydrogen and then he would have one equatorial one just like that so you could draw the tripod shapes and either the chair or boat configuration but one question is well you know what's what's more stable that's actually one of the main points of being able to visually think about the three-dimensional structure of any of these hydro any of these hydrocarbons or in this case cyclic cyclohexane so in this situation we know and from past videos that all of these carbons with their hydrogens around on these bonds these have electric electron clouds around them the electron clouds are negative and so they want to get as far away from each other they want to get as far away from each other as possible in this chair configuration you have this carbon up here this carbon the ch2 we could consider it has two hydrogen's connected to the rest of the ring it's as far as possible from this ch2 as possible so in that situation we have a lower potential energy or it is a more stable more stable shape or more stable configuration in the both configuration this ch2 up here is much closer to this ch2 I mean that's really the main difference between the two and they want to get away from each other they want to repel each other so this one will have higher potential energy or it will be less stable less stable less stable so this is just a starting point of how to visualize cyclic cyclic hydrocarbons and we'll use this information in the next video to think a little bit more about maybe the the different chair configurations that a molecule could have and what could be more stable in this situation in the case of just cyclohexane the two chair configurations are equally stable and let me just touch on that a second so you have well I don't have to actually let me see I won't copy and paste it I'll just redraw the other chair configuration for this guy actually let me just do it separately over here because I've made the colors here so confusing let me draw to the same the same cyclohexane but in two different chair configurations that it could be equilibrium in so you could have this one you could have this one so this could be one chair configuration and I'll draw it like this and then the same hydrocarbon could be an e or the same cyclohexane could be an equilibrium with this other chair configuration that looks like this that looks like this we have a little more space here so it looks like this let me do the pink goes up like that like that let me make sure I'm know I want to do it actually this pink guy goes like this and then the mid and then the blue guy is going to be just like this so notice in this situation this carbon up here is kind of at the top of the chair and this carbon is at the bottom and then they flipped but these are equally stable configurations but one thing to think about is all of the axial guys on this carbon here turned into equatorial on this carbon and vice versa on the two let me show it to you let me just draw the hydrogen's on this carbon so if I were to draw this hi this carbons hydrogen's has an axial hydrogen and has an equatorial hydrogen that would whose bond would be parallel to that right just like that and this guy would have an equatorial hydrogen whose bond is parallel to actually both of these guys and an axial hydrogen and an axial hydrogen but when it flips and I'm just drawing those guys hydrogen's but when this structure flips like that what happens well this hydrogen over here this hydrogen over here goes into this position and this yellow hydrogen over here goes into this position so over here it was equatorial and now it becomes axial and now it becomes axial in the same argument government can be made over here this equatorial hydrogen when it flips when this whole blue part flips down now becomes axial now becomes axial and this axial hydrogen when you flip it down becomes equatorial it becomes equatorial and you can actually do that for all of the hydrogen's over here you have an axial hydrogen axial hydrogen once you flip it once you flip it let's say you have an axial hydrogen and then you have an and then you had an equatorial hydrogen an equatorial hydrogen when you flip it these two equatorial hydrogen's become axial so they become axial and then both of these guys become equatorial so let me do that in yellow both this guy and this guy become equatorial so this and that become equatorial they become parallel to the other end and you could do it for this these two hydrogen's as well so that's another interesting to think about it and this is really just practice on visualizing what's going on and when we when we visualize these molecules in three dimensions