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Current time:0:00Total duration:14:38

Video transcript

in the last video we saw the two chair shapes or the two chair configurations of just plain vanilla cyclohexane we're equally stable there was nothing that would imply that this is more stable than that or vice versa but we want to do in this video is address what happens if instead of just a pure cyclohexane what if we added a methyl group to it and so instead of just a cyclohexane let's think about a methylcyclohexane and so we know what our what to do when we're naming things so a methylcyclohexane methylcyclohexane will look like that look like we'll have a hexagon here this is the way we've been drawing it historically we've been drawing it just like that and then it'll have a methyl group and you could literally just draw a methyl group like this or maybe like this and by implication you have a carbon here essentially a ch3 group here and of course you have another hydrogen bonded here now if you were to draw this methyl group in kind of three dimensions like this it would look just like our different chair positions before but instead of this H being just an H we could make this entire methyl group so we could turn this ch3 right here so this let me draw it out in a different color we could make this carbon that has implicitly has three hydrogen's on it so this is a ch3 group that is a methyl group that's right there we could turn one of these we could substitute one of these hydrogen's with it and then this would be one of the shapes of this methylcyclohexane so ch3 let me write it down just to practice our naming so this is methyl methyl cyclo hexane now in this this is one of their chair positions and then if this were to flip down and the other side were to flip up the other chair position would take this methyl group from being in an axial position and it would put it in an equatorial position so this is the same group right here let me put a circle around it it's a whole group that's the ch3 now the question we want to answer in this video is one of these two configurations going to be more stable now you might just be able to eyeball it looking at this diagram say hey when we're in this when we're in the position where the methyl group is in the axial position it's going to be closer to all of these other carbons over here closer to their electron clouds maybe it has higher potential energy maybe it'll want to spring away and if that was what you're guessing or what you're kind of eyeballing you'd be right because in this position this methyl is much further away from all of the stuff out here so they'll be less I guess you can consider it electron cloud electron cloud crowding if you will now to see that a little bit more clear I want to do a a what we call it as a double Newman diagram and really that's the whole motivation of this video to kind of expose you to that a Newman diagram isn't just useful for simple things like a butane or an ethane you can actually do it with cyclical rings and to do the Newman diagram let me number these carbons so we could number them like this so this this is carbon one two three four five six and you don't have to call this one methyl cyclohexane because what you always have whenever you have only one group attached to the ring you implicitly start the numbering at the at the carbon that the group is attached so that is the one carbon over here this is the one carbon two three four five six and over here once again is the one carbon two three four five six now what I want to do is draw a Newman draw two Newman projections and both of them will involve well actually I'll draw four but you'll see what I'm talking about in a second so the first Newman projection I'm going to start at this carbon right that carbon right over there so let's think about what that would look like so we're at the front we're staring straight on to that carbon so this carbons in the front this carbon over here this carbon carbon number two is in the back so let me label this this is carbon number one that we're looking head-on now right in the axial position you have that methyl group so let me draw that so in the axial position in the axial position you have that methyl group right over there and then in the equatorial position right here you have this hydrogen so let me draw that so you have this hydrogen and then over here this bond right over here this gets us to another ch2 group so let me draw that like this so that gets us to another ch2 that's a ch2 right there right or maybe I should Circle it like this this whole thing is a ch2 so it's bonded to the carbon but the carbon has two hydrogen's on it and this is actually carbon number six so this is the front now if we're staring straight and carbon number two is right behind it let me draw carbon number two I'll do it in this blue color so carbon number two that bigger circle now carbon number two what's going on over there it has a hydrogen in the axial position going straight down it has a hydrogen in the axial position going straight down this hydrogen right over there and then it has a another hydrogen going equatorial so this green hydrogen right here is going equatorial it looks like that and then it connects to carbon number three which is another ch2 it has two hydrogen's branching off of it so this right here so this this in front we have carbon number one in the back we have carbon number two let me color code it a little better Carbon number two and then carbon number two branches off two so if you think of this branch right here you think of that branch right here that's carbon number two branching off to carbon number three or ch3 right there let me do that in a new color so this is a actually ch2 group that's a ch2 this is a carbon it has two hydrogen's right so this is a ch2 this is carbon number three right this is carbon number three right there and then that goes then and actually I'll pause there what I'll do now is I'll draw another axial another Newman projection but for this Newman projection we'll be looking straight on we'll be looking straight on our carbon number five our carbon number five right here we're going to see we're going to form a ring because this the first Newman projection we just did essentially covers this bond this bond is sitting straight into the screen the way I did it right now carbon number two is directly behind carbon number one so I guess the opposite side of the Ring is five to four if you look at here one two is there and then five to four is just like that so you can imagine when we're doing the Newman projections we're looking straight on here the left Newman projection we're going to look straight in that direction on the right Newman projection so if we have carbon number five in front if we have carbon number five in front what are its bonds going to look like well it's going to bond to carbon number six over I'd over here so we can make this this right let me do that in a different color this right here this bond right here is this bond right here to that same ch2 that our first Newman projection bonded to and then he's going to he's going to have two hydrogen's I haven't drawn them here let me draw them just so you can see them I'm going to have two hydrogen's one in an axial position and one in an equatorial position it's hard to see now but he's going to have one hydrogen axial and one in an equatorial position and then in the back we're going to draw carbon number four and carbon number four I will do in this green color so carbon number four or actually well yeah I'll do carbon number four in that green color and this is really just an exercise in visualization that's why I wanted to do it with you so carbon number four has an axial hydrogen so it has a hydrogen pointing straight down that hydrogen is that hydrogen it has an equatorial hydrogen going out like that and then it bonds to carbon number three so this bond right here is this bond just like that so what do we see immediately when we put when we draw this chair position when our when our methyl group is in the axial position what do we see methyl methyl in axial position we see that it's go SHhhh or gosh I don't know the best way to pronounce it it's only sixty degrees away from this from carbon number three this is only sixty degrees our dihedral angle you know when you when you use a Newman projection this is only a a sixty degree angle it's sixty degree angle it is gautsche to carbon number three it is gautsche to carbon number three so maybe they're crowding each other a little bit let's compare it to the situation where our methyl group is equatorial where the carbon that it's attached to is on the down part of the chair let's see what that Newman projection looks like so same thing same thing let me scroll over to the right a little bit so this is this configuration and it's in equilibrium it's in equilibrium with this configuration right here we'll do the exact same exercise carbon number 1 but now carbon number 1 and carbon number one right here the axial the the the hydrogen is now actual and it's pointing straight down let me draw that so we have a hydrogen pointing straight down now hydrogen's pointing now down and now the ch3 is in is in a is in a equatorial position which you can see more clearly on this and over here so you have a ch3 the methyl group is right there and now this bond this bond to carbon number six will look like this so it will look like this so you have a ch2 this is number six right over there and now if we were to go to back if we were to go to the carbon number two in back which we had done in the blue color before so I'll do it in the blue color again carbon number two in black it has a hydrogen in the axial position going straight up that's that hydrogen right there has a hydrogen going straight up it has another hydrogen over here and then it bonds to carbon number three in the back so ch3 or ch2 over here so this guy is the same thing as this guy but now we've flipped configurations so he bonds to that guy in the back and now we do the Newman diagram looking straight on to carbon number five or looking actually straight on to right Carbon number five so we're going to look straight in so we're looking straight in this direction for this Newman projection now we're gonna look straight in this direction for our other Newman projection so carbon number five carbon number five if we draw it in the front it actually I haven't have I haven't drawn it here but it has a hydrogen the axial position and then it has a hydrogen in the equatorial position so carbon number five if we look at this and this isn't this is really III if you're getting a little stressed out about this because it's a little hard to understand you might want to re-watch this is really just a an example of visualization so I really hope this isn't confusing you if you find this really daunting this isn't going to really trip you up in the left rest of organic chemistry but if you can't get it it's even better it'll be that much better at visualizing some of these molecules so if we look straight on in carbon number five we have a hydrogen in the axial position coming straight down hydrogen in the axial position coming straight down it is bonded it is bonded carbon number five is bonded to carbon number six which is right over which is right over there so let me make it very clear so this bond right here now I want to do a different color this bond right here is the same thing as this one this is a really good way to if you can do this then your brain is pretty good at translating between Newman projections and these kind of seat diagrams that we have up here so this is this bond this axial hydrogen this axial hydrogen is this axial hydrogen and then we have another hydrogen we have this hydrogen right here which would be that like that and then in behind it you have carbon number four carbon number four is it's like that so carbon number four draw a circle it has a hydrogen in its axial position another hydrogen like that and then it bonds to carbon number three so this was number three right here so just like that so what do we see about this methyl group here in this situation this methyl group is anti it's you know there's two ways to think about it it's dihedral angle versus carbon number versus carbon number three is now 180 degrees over here it was gosh it was sixty degrees it was dihedral angle to carbon number through sixty degrees now it's 180 degrees so it's much further and now it's dihedral angle to the carbon number six is also it's also 120 degrees so in this situation where our methyl group is equatorial it's not its axial here its equatorial here when we jumped back down because notice it's parallel the bond is parallel to parts of the Ring in this situation we are farther away from the other methyl groups there's less crowding and so this is a more stable situation so you could say it is anti is an anti configuration configure a configuration relative to carbon number three while over here it was gauche to carbon number three i don't know if i'm saying it's go chicago which i don't know the best way to pronounce it so in this case there's less crowding this is more stable lower potential and lower potential energy more stable so hope you found that interesting this was really a way just an exercise in being able to go from this visualization to kind of this double Newman diagram and if it makes it any easier the way you could think about it is we're kind of viewing we're viewing in this Newman diagram right here where this carbon number six is this carbon number six this ch2 is this one back there so when we're looking at it from this we're kind of looking at this this hexane ring from this direction we see this in front this is this and we see that and back that is that over here same thing we're looking at it we're looking at it directly we're looking at it directly from we're looking at it directly from this direction we see this guy on top is over here and this guy on the bottom is over here so hopefully that helps your visualization a little bit