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Current time:0:00Total duration:14:22

Video transcript

in the video on sp3 hybridised orbitals we went in pretty good detail about how a methane molecule looks but just as a bit of review it's a tetrahedral shape you have a carbon in the middle and then you would have a hydrogen you could imagine I'm drawing it like this because this hydrogen is poking out of the page then maybe you have another hydrogen that's in the page you have one above the carbon and then you have one that's behind the page so you could imagine it's like a it's like a tripod it's like a tripod with a pole sticking out at the top of the tripod or if you were to imagine the shape another way if you were to connect the hydrogen's if you were to connect the hydrogen's you would have a four-sided pyramid with a triangle as each of the side so it would look something like this it would look it would look something like trying my best to draw the pyramid would look something like this if you could see through it so this would be one side another side would be over here and then the backside would be over here and then the fourth side is actually the side that's transparent out front so the fourth side the fourth side would be the actual kind of thing that we're looking through when we look at this pyramid it would be this front side right over here so you could imagine in different different ways this was the case with methane this is the case with methane now let's extend this into a slightly more complex molecule and that's ethane so the way we've been drawing it so far I guess the simplest way to draw ethane is just like that by implication this is ethane by implication you have a carbon there and a carbon there and they'll each have three hydrogen's bonded to it and we've drawn it something like this three hydrogen's bonded to each of these guys but now we know that carbon has these sp3 hybridized orbitals that it likes to form more of a tetrahedral shape when it bonds so an ethane molecule would actually look more like this let me draw the carbons so I'll do the carbons in orange so if that's the carbon and that's the carbon so you can imagine you have a carbon molecule here I'll draw it is this little circle and then if we have some perspective so the carbon carbon bond is going to look like that and then you have another carbon molecule right over there so that's that bond over here and we want both the carbons all of their bonds to be kind of in a tetrahedral shape so then you can imagine this bond over here going like this this bond going like that and you have your hydrogen at the end when the let's make the green circles the hydrogen's so you have that hydrogen and then or actually just the the circles will call them hydrogen and then you could imagine this one maybe it's coming out of the page a little bit that is that hydrogen that is that hydrogen let me label the hydrogen's actually I mean all different colors you can see what I'm talking about and then this hydrogen is going right below it may be pointed back a little bit so that hydrogen is right over there so you can see this carbon it has a its bonds have a tetrahedral shape or if you just looked at this part of it these would be the base of the tripod and this would be the thing sticking up now for this car but it'd be a very similar idea a very similar idea this hydrogen right here this hydrogen right here might be sticking down like this and then and I'll stop switching colors soon enough it takes a lot of time that hydrogen over there it's pointing out in that direction like that and then you would have let's see what colors do I have left well I'll just do yellow that hydrogen right there maybe it's pointing out like that so this is a possible configuration for ethane and the way that I've drawn this right now and you can actually have a model that has this where you have little you know wooden sticks with balls in this that balls represent the actual atoms this is called a ball and a ball and stick model ball and stick model and this is a ball and stick model for ethane now simpler way we could have drawn this this is called a horseshoe projection or actually the sawhorse production I would say horseshoe a sawhorse projection it would look like this that's the exact same configuration of ethane and a sawhorse projection and you know what a sawhorse looks like I saw well it looks like what I'm about to draw it looks like this so you could well I could I could draw it exactly the way I drew it here so you have the carbon carbon and then you have the hydrogen hydrogen hydrogen and then you have a the way I've drawn it well the way I've drawn it up here is more like this just so we see the parallel and then we can rotate things around the way I drew it up here you have a hydrogen hydrogen and hydrogen and then over here you have a hydrogen hydrogen and hydrogen this is a sawhorse sawhorse projection now either way you depict it I mean these are really the same way this is kind of just like the laser way of doing it on some level you're not drawing all of these circles and all of that and you're putting a little less care into actually showing the angle how things are angled away from the carbon and showing the tetrahedral shape but in either case when you start visualizing the molecule in this way you start to realize there's there's well there's actually an infinite ways that these things can be configured and that all comes from that all comes from the notion that this is just a sigma bond right here we learned that in the video on sp3 hybridization in Sigma and PI bonds this is just a sigma bond sigma bond and so we can rotate around the bonds one of these carbons could rotate around the kind of the axis of that bond without the other carbon having to necessarily rotate with it if this was a double bond if this was a pi bond they would have to rotate together so you could have a situation like I've drawn here or you could have a situation where they're kind of rotated the inverse of each other this is what I mean so I'll do a ball-and-stick so let's say this is our front carbon that is our back carbon and we'll compare it to this one over here so let me draw this guy the exact same way so he's got a hydrogen down here he's got a hydrogen down there he's got a hydrogen up there and then he's got a hydrogen up here and he's got a hydrogen up here so that part of the ethane looks identical now what I'm going to do is I'm going to flip the other side of the ethane I don't want you to play close attention because hopefully you'll see the difference between the two so instead of during this blue ethane down here I'm going to drew this blue hydrogen down here I'm going to do it on top so this blue one I want to do that in blue this blue hydrogen put it on top so I'm just rotating this around so if the blue hydrogen's on top the if I've rotated it so the blue ones on top now and now the green one the green one is going to go over here so now the green hydrogen is now over here and then this purple or this magenta hydrogen the way I've rotated is now going to go over here is now going to go over there so what's the difference between this configuration and this configuration right here and we could have had every other configuration in between but what's the real difference here well here the hydrogen's are all you could imagine that hydrogen is kind of if you were if you were looking from that direction that hydrogen is directly behind that hydrogen that hydrogen is directly behind that hydrogen that hydrogen is directly behind that this is called an eclipsed configuration or an eclipsed conformation so it'd be clips eclipsed conformation conformation and this right here nothing is behind anything if you went straight back from this guy you'd get to this point and no one's behind it and if you went straight forward from this guy you'll get so in no way are any of the guys in the back if once again if you're viewing from this direction are they blocked by any of the people here so we call this a staggered conformation staggered conformation now why do we even care you know okay I can twist around this black this this this this back molecule what's that even going to do for our actual you know why does it even matter well one it's you know it's just interesting that you can actually change that this thing can twist around without changing the front without the front molecule having to twist with it but even more important these have different energy levels so you can kind of think think of them as you're kind of twisting a spring and the spring might want to go back to one conformation or another and to visualize it a little better I'll draw what's called a Newman projection a Newman projection so I'm going to draw this exact thing but with a Newman projection you draw the carbon molecules directly in front or directly behind each other so the so in this situation you would draw the carbon molecule in front would just be the intersection of these bonds so for the new more a Newman projection let me draw that out so it's a Newman Newman projection and I'll start with the Newman projection for the staggered conformation so in the front will consider this carbon the front carbon we have our hydrogen pointing straight down we have our hydrogen pointing straight down like that and we have a hydrogen coming out to the top left like that and then we have this hydrogen over here coming up to the right I want to do that in that same color so the front carbon is implicitly into the intersection of the of the bonds of these three hydrogen and then the back carbon the back carbon I said the front carbon is the intersection of the bonds of these three hydrogen's the back carbon you represent it as a circle so this circle represents the back carbon the front carbon is kind of that point there just just a way of visualizing it but if we were to draw it this way the back carbon now has that blue hydrogen popping off of it so it has that blue hydrogen so it would look like so it has that blue hydrogen this green hydrogen that green hydrogen and then this magenta hydrogen and then you have this magenta hydrogen and when you look at it like this it is more clear that it's staggered we're just looking straight on to this ethane molecule we look straight on the front carbon they're obviously blocking each other but this way you can see the front carbons hydrogen's are staggered relative to the back one so this is right here once again this is staggered now let's draw the eclipsed conformation as a Newman projection so as a Newman projection the front is going to look the same you have you have a hydrogen there you have this hydrogen if that hydrogen and then you have that blue or that I guess that purple hydrogen down here so that's the front of it but the back there right behind it so let me draw the back carbon from the front carbon is kind of represented by just that dot the back carbon the back carbon will represent like that and the staggered confirmation if you if this guy is really behind that you're gonna have to draw it like like right there like right behind it but since that's a little bit messy normally when people draw a staggered projection staggered up a eclipsed conformation as a Newman projection instead of directly eclipsing that last that back hydrogen they'll put it to the right a little or they'll stack they'll push it off a little bit so that's that hydrogen that magenta hydrogen is right there it's really right behind the front one but they're just this is just so you can actually see it that it's there and then finally you have this blue one down there so that blue one is going to be right right over here so this is the Eclipse this is the eclipsed conformation as a Newman projection eclipsed conformation conformation and as you can see it's eclipsed the back hydrogen's are eclipsed by the front ones if I were to draw it perfectly they would be right behind it now there's one other piece of I guess one more idea I want to introduce you to and that's the notion of the angle between the different hydrogen so if you wanted to say well what is this angle what is this angle between the blue hydrogen and this pink hydrogen right here now you know when you actually think of it in three dimensions it's like wow you know you can't really say the angle over here between the blue and the pink but when you on a Newman projection when you're just saying how much are they rotated away from each other this angle right here is called a dihedral angle dihedral dihedral angle sometimes it's just said you know this this hydrogen relative that hydrogen has a DA of in this case 60 degrees in this case this hydrogen relative that hydrogen has a dihedral angle of 0 degrees and it's a way of saying how staggered or how eclipsed you are now one last thing I touched on the idea so again why do we even care well all of these hydrogen's have electron clouds around them and all of these bonds have electron clouds around them and electron clouds they're all negative so they want to get as far away from each other as possible they're all stable now because they've bonded in ways that they have you know nice stable structures everyone feels like they have full valence shells full orbitals and so the electron crowds want to get away from each other now in this situation in the Eclipse conformation in this and the Eclipse conformation this hydrogen and this hydrogen let me do it in let me do this hydrogen and this hydrogen are as are closer to each other then when you go to the staggered conformation the staggered conformation the closest hydrogen to this guy is going to be either that hydrogen or that hydrogen but they're both further away then this hydrogen was in the eclipsed conformation so in general the staggered conformation is going to be more stable it's going to have a lower potential energy you can imagine that if you start with an eclipsed conformation these guys are all want to get get away from each other so it's kind of like this is the wound this is the wound conformation it has higher potential energy so you want to unwind and will want to unwind to a staggered conformation because in this conformation all of the hydrogen's have gotten as far away from each other as they can get