4.3 Newman Projections

So if you're gonna understand Newman projections I highly suggest you build a model similar to this one right here. So rather than looking at it from a normal perspective here: so if you look here this hydrogen right here is coming out at you, it's a wedge. And this one's coming out at you as well, as a wedge. And these two back here are both corresponding to dashes, they're going away from you. And then the methyl group here and the methyl group here both in the plane. What we're gonna do is we're gonna turn this molecule 90 degrees and give ourselves a little bit different perspective. And this is the perspective we're looking at when we look at a Newman projection. So in this case this would correspond to what's called the anti-conformation. It's a special type of staggered conformation and you can see why they call it staggered because the three bonds coming off the front carbon that I can see are exactly in between the three bonds coming off the back carbon that I can see. So hence that's a staggered conformation. N ow there's an infinite number of possible combinations because we can just start rotating this one degree, one degree, one degree, one degree, you know at a time. And so there's an infinite number of combinations and out of that infinite number of possible conformations we only really draw six of them and we go to the extremes. That would be the high energy and low energy extremes. And the staggered confirmations here are the lower energy extremes. So and we're gonna start rotating this 60 degrees at a time till we get back to this anti-conformation. So as we rotate it sixty degrees apart we get to our first eclipsed conformation. You can see why they call it an eclipsed conformation. The front carbon's three bonds are exactly in front of the back carbon's three bonds hence- eclipsing. So and this ends up being higher energy and the reason it's being higher energy is: the atoms are as close as they could possibly be together, so and the bigger the atoms the more they'd be bumping into each other- we call it steric hindrance- but also the electrons and the bonds are as close as they would ever be together and electrons being negatively charged- that's a repulsion. That's high energy as well. So and again the atoms being near each other called steric hindrance. So the bonds and the electrons repelling each other is called torsional strain. So steric hindrance or steric strain and then torsional strain as well. And those are the two reasons why these eclipsing interactions are the highest energy because we're gonna have the greatest amount of steric and torsional strain. So if we rotated another 60 degrees we're gonna be back to another- so I went over a little bit there- but back to another staggered conformation. So in this case this staggered is not near as good as the anti-conformation we had just a second ago and that's because this carbon and this carbon are now only 60 degrees apart. So in being only 60 degrees apart, we call that a gauche interaction. And so there's more steric hindrances associated with these gauche interactions than we had in the anti-conformation. And in this case the bigger these groups are in the gauche interactions the higher energy they are. And so often times we'll rank different Newman projections from all he'll based on how many gauche interactions it has, as well as how large are the groups that are involved in these gauche interactions. But keep in mind these gauche interactions are only ever in a staggered conformation. We'd never talk about them in an eclipsed conformation. If we rotate another 60 degrees clockwise here, we're back to another eclipsed conformation. And this one's higher energy not as stable as the last one we had as well- the last eclipsed. And in this case, because these two large methyl groups are now the ones eclipsing each other. The bigger the groups eclipsing each other the higher energy as well. So not all eclipsed are equivalent and not all staggered are equivalent. Let's rotate it another 60 degrees. So and now we're to another staggered conformation and yet again we have another gauche interaction between the methyl groups. So rotate it another 60 degrees, we're back to another eclipsed conformation, not as bad as the last one. Equal-equivalent energy to the first one we showed. And then finally rotating it back another 60 degrees gets us back to our lowest energy, most stable conformation: the anti conformation. In this case a special staggered conformation. So this is what I- you know- kind of the understanding you need. It's helpful if you see it in a model and hopefully this helps but let's draw some pictures. So here you're being asked to draw the lowest energy conformation of 2-chlorobutane as depicted below here. So real important here it's down the C2 C3 axis. We define it like that- we're telling you which bond to look down. In this case when we define C2 and C3, it's the numbering system you'd use if you were naming the compound. So in this case with 2-chlorobutane, we'd number it 1, 2, 3, 4. Not because we always number things left to right but because- the chlorine being the only substituent- would get the lower number if we number left to right not right to left. If I had had this reversed you'd actually want to number the other way around, and that would affect which side of the molecule you're looking at. Which is really important as we'll see here in a minute. But in this case being a lowest energy conformation we should first of all realize that we want to draw a staggered conformation, not an Eclipse conformation. So I'm here looking down the C2-C3 axis. We're gonna position our eyeball here to look right down the bond axis right there. So carbon 2 here's gonna be our front carbon, depicted by a point. Carbon 3 would be the back carbon depicted by the circle in our Newman projection here. So in this case our point for the front carbon, we can see the bond to the chlorine. We can see the bond to this methyl group right here and then there's a hydrogen not drawn in that I'll draw in that we'd also be able to see. But the bond between 2 & 3, that's the one we can't see cuz we're looking down that bond axis. Carbon 3 is right behind carbon 2 in this case. And in this case we've got an option. We can draw our bond in the plane straight up or straight down. So and that's this guy here so everything that's a wedge from this side of the molecule is gonna appear on our right hand side and everything that's a dash is gonna appear on our left hand side. But if you're not a wedge or a dash you're right down the middle. In this case it's right down the middle, right below where we're looking, so it's going to be straight down. So and then go off 120 degrees in both directions- we'd have 2 more- are the locations. So this is your CH3. So your wedge coming out of the paper here or out of the monitor, however you want to look at this, would be your chlorine and then going away hydrogen would be on your left being behind your monitor, looking at it from that side view. So then the back carbon, we can't see it. So we draw a circle to kind of represent that we can't see it and in this case he's got a couple of hydrogens that formerly weren't drawn in. I will just add to the diagram. One's a wedge, one's a dash. And then he himself also has a methyl group here, carbon number 4, as well. And again the wedge hydrogens are on our right, the dashed hydrogens are on our left but the methyl group it's not on the right or the left since it's in the plane. And from that sideways perspective it should be right above where we're looking. So we've got 2 CH3s there and then those hydrogens, one on the right: that's a wedge, and one on the left: that's a dash. So and here's one of the staggered confirmations of our molecule. Now in this case this is one of three staggered confirmations we could possibly draw. So it's a great place to start. We wanted the lowest energy one. So it's one of those three staggers. And if we look here we have a gauche interaction. So in this case we happen to have a gauche interaction. So between the methyl group and the chlorine. So if we envision the other three staggered confirmations, in fact, let's just draw one of them. I'm going to keep the front carbon fixed and it's the back carbon I'm gonna rotate relative to that front one. I could have done the exact opposite and kept the back carbon fixed, rotated the front one. So in our case, I'm gonna rotate that back carbon 120 degrees clockwise. And so all three of those groups are gonna trade places. So the methyl groups are now going to be down your lower right. So hydrogen here, hydrogen here, and if we look now we have another gauche interaction right here. And the question really becomes which gauche interaction is preferred? So and it turns out chlorine takes up less volume overall than CH3 and has less of a steric problem than your methyl groups. And so here, this gauche between the methyl and the chlorine is better than the gauche over here between both these methyl groups. And so this one so far is lower energy and we don't actually, technically, have to write the last one. If you just envision rotating around another hundred and twenty degrees, you'd find out that this methyl group, that were having a problem with, would be in this position and there'd still be this gauche interaction that would show up in the next one. Right- you're right-right like here-so we're gonna end up with one gauche interaction no matter what. Usually you want fewer gauche and then if you have the same number then you want gauche with smaller substituents. And again chlorine being smaller than the methyl group- this one is the lowest energy staggered conformation that we come up with. The overall lowest energy conformation for the 2-chlorobutane depicted below.