Newman projection practice 2
How to determine the most and least stable conformations using Newman projections.
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- why carbon 3 has a methyle group while 4 has 2 hydrogens?? how am i suppose to know whether to put a methyle group or a hydrogen?(5 votes)
- If you look at the bond-line structure, you see that C-3 has a methyl group (the wedge).
C-4 has two hydrogens. We don't usually show them explicitly in bond-line structures, but they must be there to make up the four valences of carbon.(4 votes)
- At6:18, Jay said it doesn't matter whether you rotate the front or back carbon. Can anyone explain to me why?(4 votes)
- Because you would end up with the same molecular conformations. If you were to flip the whole molecule around, like so what was the back carbon goes to the front, you would have the same molecule.(6 votes)
- At9:36, how can we conclude that the second conformation has a higher energy than the first one? Without calculating and comparing the total energy of methyl-ethyl gauche interaction and hydrogen-methyl gauche interaction, with, that of methyl-methyl gauche interaction and ethyl-hydrogen gauche interaction, how can we tell which one is least stable?(3 votes)
- The conformations shown at9:36are eclipsed conformations.
You do have to add all the individual repulsions to get the total strain energies, but the Et-Et repulsions are the main factor.
Assume that the repulsion energies (in kJ/mol) are:
H-H = 4; H-Me = 6; H-Et = 8; Me-Me = 11; Me-Et = 13; Et-Et = 15.
Which conformer do you calculate to be the least stable?(5 votes)
- What is the torsional strain between two eclipsed ethyl groups?(3 votes)
- Why does the methyl group has to be in carbon 3? Why not carbon 4 or the others?(2 votes)
- It's just the example he came up with. The first diagram had the methyl on carbon 3 so it's on carbon 3. Also if it was on carbon 4, the carbon 4 would actually become carbon 3 anyway because of the nomenclature rules.(2 votes)
- How do you know if eclipsed or staggered is more or less stable? does it depend on the molecule or is staggered always more stable?(1 vote)
- A staggered conformation is always more stable. For more detail, watch the first couple videos in this tutorial, which are about the basics of staggered and eclipsed conformations. :)(3 votes)
- At7:08, how is there only one gauche interaction? Can gauche interactions not be formed with hydrogens?(2 votes)
- Why aren't there Gauche interactions between hydrogens? Do they only exist between bigger groups like methyl and ethyl?(2 votes)
- Does it matter which carbon is on top while we draw/view a Newman projection?(1 vote)
- how do you know which hydrogens will be coming out and which will be going away from us in space?(1 vote)
- [Voiceover] Let's get some more practice with Newman projections. So, for this compound, we're gonna look down the c three c four bond and draw the most stable conformation. So let's start by numbering our carbons. This must be carbon one, two, three, four, five, and six, and if we look down the c three c four bond, that's this bond right here, we're gonna put our eye along this axis. Let me draw an eye in here, so we're going to stare down this way and draw what we see. Well I'll show you in a video in a minute what we would actually see, but it's very important to be able to draw these Newman projections without the use of a model. So let's start by thinking about what is attached to this carbon three here. There's a methyl group coming out at us in space, which means there must also be a hydrogen going away from us in space, and what is attached to carbon four? There must be a hydrogen coming out at us in space, so a hydrogen here, and a hydrogen going away from us in space, so now that we've drawn that in, we can start to draw our Newman projection. And we start with a point or a dot to represent carbon three, so that point there is carbon three. What is attached to carbon three? Well, there is a methyl group that would be going down and to the right, so if your eye is here, you'll see a methyl group going down and to the right, so let's draw in a C H three down and to the right like that and then we also have a hydrogen, so this hydrogen will be going down and to the left, so let's draw in that hydrogen down and to the left. Next, we would have a C H two, C H three, or an ethyl group and this would actually be going straight up if we're looking at it from this perspective, so this would be going straight up, so let's right C H two, C H three. Next, we need to think about carbon four, so we're thinking about this carbon right here. We wouldn't be able to see it because carbon three would be in the way, but we know that carbon four is there and we represent carbon four with a circle on our Newman projection. What is attached to carbon four? Well we know that we have a hydrogen going up and to the right, so let's draw that in, so there's a hydrogen going up and to the right attached to carbon four. There's also, there's also, a hydrogen going up and to the left attached to carbon four, so let's draw that in on our Newman projection. And then, finally, we would have a C H two, C H three, and this would be going down, so a C H two, C H three going down, so let's draw that in, C H two, C H three. We ll it gets annoying to draw in all these C H two's and C H three's, so let's, let's redraw this Newman projection. Let's say that C H two, C H three, we know that's an ethyl group, so let's abbreviate that with Et, and a methyl group, let's just say that's Me, and then we have our hydrogen right here, and then for our back carbon, we have a hydrogen here, we have a hydrogen here, and then we have an ethyl group going straight down. It's just a little bit easier to see this way, and in the video, I'm gonna make an ethyl group red, so you're gonna see a red, a red circle, a red sphere for an ethyl group in the video, and I'll make the methyl group blue, so it's easier to show different conformations, if you just represent it by a sphere, and you'll see what I mean, and from the video, we're gonna figure out the most stable conformation. We know that has to be a staggered conformation, from earlier videos, so we'll look at all the different staggered conformations, and we'll pick which one is the most stable. Here we have carbon one, and then carbon two, and then carbon three, notice there's a methyl group coming out at us in space, attached to carbon three. Then we have carbon four, and we're gonna stare down the carbon three-four bonds, so let's rotate the molecule here and let's stare down the c three, c four bonds, and notice we have a staggered conformation. Up here, we have an ethyl group. On the right here, we have a methyl group, and then down here, we have another ethyl group, so hopefully you see the staggered conformation. Remember that red is an ethyl group, so here's an ethyl group, and here's an ethyl group, and blue represents the methyl group. It's just easier to work with the model set this way, so we're going for staggered conformations, so if I rotate the front carbon and keep the back carbon, keep the back carbon stationary, we get another staggered conformation, and if I rotate again, then we get another staggered conformation. Hopefully you can see the Newman projection that we drew matches the picture from the video, and this is the same Newman projection. I just made the ethyl groups red and the methyl group blue. In the video we moved the front carbon, we rotated the front carbon, we held the back carbon stationary, so this ethyl group in red would move over to this position. The methyl group in blue would move over to this position, and finally, this hydrogen right here, I'll make it green, would move over to this position, so that's this hydrogen in green. Let's go ahead and draw the next conformation, the next staggered conformation. If we held the back carbon stationary, we can go ahead and draw in the back carbon and what's attached to the back carbon, these hydrogens and this ethyl group, and next, the ethyl group in red, on the front carbon moved over to this position. The methyl group in blue moved over to this position and the hydrogen in green moved over to this position, so we can see that matches what we have, what we have for our picture down here. We have our two ethyl groups are now gauched to each other and then we have our methyl group over here in blue, and our hydrogen in green, let me highlight that. The hydrogen in green is this hydrogen. We can rotate one more time to get our last staggered conformation. Our ethyl group in red can rotate over here. The methyl group in blue could rotate over here, and that would mean the hydrogen in green has to rotate over to this position. So then draw in the back carbon with the hydrogens and then the ethyl group. It doesn't matter if you rotate the front or the back carbon. I chose to rotate the front carbon here, and that would move our ethyl group over to this position. So now our ethyl group is here. The methyl group in blue would move up to here, and the hydrogen in green would move over to here, so hopefully we can see that. Let me highlight everything. So here's an ethyl group, all right, so down here we can see our ethyl groups, our methyl group in blue, and finally, our hydrogen in green, so now we're finally able to choose the most stable conformation out of our three staggered ones here. So we need to think about the gauche interactions that are present, and we'll start with this conformation on the right. Here, we have an ethyl-ethyl gauche interaction, and ethyl groups are pretty bulky, so this gauche interaction would destabilize this conformation. Let's look at this conformation next. We have an ethyl-ethyl gauche interaction, and we also have a methyl-ethyl gauche interaction, so we have two gauche interactions, so that means that this conformation is even more unstable and finally, we have this one right here. We have only one gauche interaction and it's between an ethyl group and a methyl group, and since this methyl group is not as bulky as another ethyl group, that means this is the lowest energy conformation. This is the most stable. In part b, our goal is to draw the least stable conformation or the one highest in energy, and that must be an eclipsed conformation, so let's go to the video, and let's start with the staggered, and then go to an eclipsed conformation, and from that eclipsed conformation, we'll look at the others and we'll choose the one that's the highest energy. So here we have our staggered conformation. If I rotate the front carbon, we see one eclipsed conformation. I can rotate again to get another eclipsed conformation, so here's another one. You can see this one has the two ethyl groups really close together. I can rotate again to get our final eclipsed conformation. Here we have pictures of the three eclipsed conformations from the video, and to save time, let's just analyze the pictures and then we'll draw the least stable conformation as a Newman projection. Let's start with the conformation on the right. We can see we have an ethyl group eclipsing a hydrogen, a methyl group eclipsing a hydrogen, and a hydrogen eclipsing an ethyl group, so we don't have any of the alkyl groups eclipsing each other, so this one, this one is definitely not the least stable. We're looking for bulky groups interfering with each other, so steric hindrance. Let's move over here to this conformation. We have two hydrogens eclipsing each other, we have an ethyl group eclipsing a hydrogen, and then we have a methyl group eclipsing an ethyl group, so there's a source of some strain, so that's gonna increase in energy, so this conformation is definitely higher in energy than this conformation, but let's compare this one to our center one. For the center one, we have two very bulky ethyl groups, so an ethyl group eclipsing another ethyl group, and that is very unstable, right? This increases the energy. That's a lot of steric strain, so this, this is the least stable conformation. These ethyl groups want to be as far away from each other as possible, and here we've put them very close together in space, so let's go ahead and draw the Newman projection for this conformation, and we start with this carbon, so that carbon is represented by a point, and attached to that carbon we have a methyl group going up and to the left. We have a hydrogen going up and to the right, and we have an ethyl group going down. For the back carbon, so here's the circle representing the back carbon, we have a hydrogen going up and to the left, so there's our hydrogen. We have another hydrogen back here, so I'll draw that one, and then finally, we have an ethyl group going down. So this is our least stable conformation for our compound, drawn as an eclipsed conformation in a Newman projection.