Newman projections 2
Newman Projections 2. Created by Sal Khan.
Want to join the conversation?
- what is a dihedral angle?(6 votes)
- In the context of the video, a dihedral angle is the angle between two points on two different parallel circles or planes. For example, imagine a clock. The angle between the hands is a dihedral angle because the minute hand has to pass over the hour hand (indicating that they are not on the same plane, if they were they would've collided).(15 votes)
- Why wouldn't there be another confirmation where the back CH3 group is eclipsed by a hydrogen and the front CH3 group is eclipsing a hydrogen?(3 votes)
- There is. He just didn't show them all in the video.
Just as there are three staggered conformations, there are three eclipsed conformations.
One has the two CH₃ groups eclipsed. The other two have CH₃ eclipsed with H.(6 votes)
- The angles between the hydrogen atoms and the carbon atom in ethane are 109.25 degrees since it is sp3 hybridised .then dihedral angles should have been 109.25 divided by 2 i.e 54.6 degrees .then how is the dihedral angle 60 degrees???(3 votes)
- In the case of the sp3 you are looking at 4 atoms in a tetrahedral (3D) configuration, which it turns out the farthest apart they can all be in 109.25 (think a triangular pyramid) On the Newman projection you are projecting the object into a 2D plane which will give you a 360 degree circle with 3 atoms in view, which will put them 120 degrees apart, add 3 more atoms and they are now 60 degrees apart (the 4th atom in the tetrahedrons, C, is behind the other C so it is not in the "circle" of the projection)(5 votes)
- I was hoping this video would touch on this, but it didn't:
If you have an electronegative atom, like Cl or F, would the eclipsed conformation of two Cl or two F have a higher potential energy or a lower potential energy then the eclipsed conformation of two methyl groups? And if so why?
Thank you!(5 votes)
- At09:08, Sal mentions the Gauche conformation to be 2nd most stable. I was wondering which conformation would be more stable: 1. Gauche where there is a staggered state, but the methyl groups are close to each other and hence more electronic repulsions, or 2. Dihedral angle between the methyl groups is 120 degrees, there is interaction b/w methyl and hydrogen twice but hydrogen has very less electron density as compared to methyl. Thanks!(2 votes)
- The energies of interaction are approximately
• 3.3 kJ/mol for each CH₃-CH₃ gauche
• 4.2 kJ/mol for each H-H eclipsed
• 5.0 kJ/mol for each CH₃-H eclipsed
• 23 kJ/mol for each CH₃-CH₃ eclipsed
The major interactions are those between CH₃ and CH₃.
The CH₃-CH₃ eclipsed conformer is the least stable.
That is the case as long as the CH₃ groups are within about 30 ° of each other.
Then comes the CH₃-H eclipsed conformer (CH₃ groups at 120 °). It is still high-energy but more stable than the CH₃-CH eclipsed conformer.(3 votes)
- At2:25, when draws the 3d structure of butane, how does he decide that H will go up and CH3 would go down?(1 vote)
- It's an arbitrary choice.
But the CH₃ looked "down" in the original structure, so he put it down in his saw-horse projection.
It is convenient to draw the biggest groups either up or down in saw-horse and Newman projections, so most chemists do that automatically without even thinking.(4 votes)
- If we rotated the back methyl group by 60 degrees as Sal was initially saying, wouldn't that be more stable than the Gauche conformation ?? And would that be a Gauche conformation ?? Thanks !! :)(2 votes)
- In that conformation, it is still eclipsed. However, the back methyl group (in this context) is only eclipsed by a single hydrogen, which has a much smaller electron cloud than a methyl group.(2 votes)
- When he first draws the butane molecule at around the 1 minute mark, does the drawing have to be exactly like that? For example, when he draws the hydrogens for the middle 2 carbons, do they have to be both at the top or both at the bottom? If so, why? I was just thinking that they would be as far away as possible because the electrons want to repel each other.(1 vote)
- They don't have to be at any place, no. Think of them as in constant rotation but spending the majority of the rotational time in the more stable positions.(3 votes)
- How do you know when to rotate from the front or the back?(1 vote)
- You can rotate either one. The important point is the anti/gauche/staggered/eclipsed relationship of the groups to each other.(2 votes)
- How would you go the other way around? For example, if you had that newman projection, how would you know how to name it?(1 vote)
- The front carbon has a CH₃ group and two H atoms. That part of the molecule is CH₃CH₂—.
The hidden carbon has two H atoms and a CH₃. That part of the molecule must be –CH₂CH₃.
Put them together, and you have CH₃CH₂—CH₂CH₃.
There are four carbons in a row with no substituents, so the name is butane.(2 votes)
In the last video, we visualized an ethane molecule with a Newman Projection. What I want to do in this video is show that you can really visualize longer chains, or even, we'll see in future videos, even cyclical, ring-based carbon molecules with Newman Projections as well. And I guess the next most complex molecule to study would be butane. We could do propane, but butane will be interesting. This was ethane right here, butane will have four carbons. And if I were to draw it in kind of a ball and stick model, it would look like something like this. So this would be one carbon right there. Then you would have another carbon right over there, and another carbon right over there. And then you'd have your fourth carbon. And then your hydrogens. You would have a hydrogen coming out like this, like that, and then up like that. This guy would have two hydrogens that would stick out like that. This guy would have two hydrogens that stick out like that. And then finally this guy will also have three hydrogens, the ch3, just like that. Now, if we try to draw a Newman Projection, here's like, well, what do you consider the front, or the back, carbon in all of that. And you actually can pick. And what's interesting in a butane molecule is, if you pick this guy, so this is a one, two, three, four carbon, if you pick the two carbon as our front, and our three carbon as our back, and then we viewed this carbon, the ch3 as kind of one of the add-ons on to that carbon, you can then do a Newman Projection. So let's try to do this. So this'll be the front one. So we'll put this carbon in the front, and we'll put this carbon over here, we'll put this carbon over here in the back. And before I even draw the Newman Projection, let me redraw this. But I'm just going to draw this, instead of with the hydrogens, the bonds, explicitly defined, I'm just going to call this a ch3. So let me redraw this. So I'll do this in orange. So you have this carbon. I'll do this as kind of as a modified ball and stick. So that carbon, it has a hydrogen, it has that hydrogen and that hydrogen. And instead of drawing this out, I'm going to just draw this whole thing right here and I'll do it in, I'll do it in magenta. I'm going to draw this whole thing as just a ch3. So I'm going to draw this whole thing as just a ch3. So I'll draw it really big, because it's not just one atom, it's four atoms. So this is our ch3. And, well, to do ball and stick everything really should be a ball, so I'll draw a ball there, a ball there. So that's our carbon number two, and then it has this bond over here to this carbon number three. Which, when we do our Newman Projection, we'll put in the back. So our carbon number three is like that. And then the carbon number three, it has two hydrogens, and then it has this. You can kind of view it as this methyl group attached to it, if you want. It has this ch3 attached to it right there. So I'll do the ch3, I'll do it in this blue color. And so we could draw it like this. So the ch3 is coming off-- so I'll draw it really big because it's not just one atom. So it's a ch3. And then you have your two hydrogens and they're-- sorry, you have your two hydrogens down here. So let me be very clear here. This hydrogen and that hydrogen, that's that hydrogen and that hydrogen, this thing here is that thing there. That big ball right there is this whole ball. And then let me find a-- I'll do green. This hydrogen and this hydrogen is this hydrogen and this hydrogen. And when you look at it this way, now you say, oh, now I can see how I would draw a Newman Projection. I put this in the front, that in the back. I treat this whole part of the molecule as just a group, if you will. It is a group. So lets do the Newman Projection here. And we can think about where it's most stable. So the way I've drawn it right here-- I'll do this as the front, this carbon two is going to be the front molecule. So you have the ch3 group going down. And then you have these two hydrogens, hydrogen, and hydrogen. That's the front. And in the back you have this blue one. You can imagine in the front, if we want to, maybe I'll do a little small orange thing to show this is the orange carbon. And then the blue carbon is going to be in the back. The blue carbon is in the back, I'll draw it like this. So that's my blue carbon. The way we've done it here, we have a ch3 pointing straight up, and then we have our two hydrogens. Now, just like we talked about in the first video on Newman Projections, all of these groups have-- these hydrogens have electron clouds around them. This whole ch3 group has a larger electron cloud around it. It's a carbon atom plus hydrogens. They all want to get away from each other. The ch3 is even a bigger molecule. So to some degree, it's going to play a bigger role in whether something has a higher or lower potential energy or whether it's wound or not. So I guess the most obvious, or maybe it's not obvious, but the ch3 groups, since they have the biggest electron crowd, they're kind of crowding the molecule. This ch3 group and this ch3 group, they're going to want to get as far away from each other as possible. So the way we did this, it looks kind of like our staggered conformation, but when we're dealing with actual methyl groups that are separated as far as they can from each other, we call this the anti-conformation right here. And if we think about dihedral angles between the two methyl groups, the dihedral angle here is 180 degrees. 180 degrees dihedral angle. And this is the lowest potential energy or the most stable. And if that confuses you when I talk about lowest potential energy, just think about it. A rock on the ground has a lower potential energy than a rock that is 50 feet in the air. A rock on the ground is also more stable. It's less likely to do something. Something 50 feet in the air, maybe if you nudge it a little bit, it'll fall off the cliff or wherever it is. Or maybe it's already falling. Who knows? It's going to move when you have higher potential energy. Or it takes very little for it to release energy. But when you have lower potential energy, you're more stable. So this is the most stable conformation. Now what are the other situations you could do here? Well, you could keep rotating these-- let's say we rotated the back carbon around clockwise, what are the other conformations we could get? And so let me just draw the front portion right here. So you have your ch3, and then you have your two hydrogens, hydrogen, and hydrogen. And let me copy and paste this. So there's two other real-- I mean, there's everything in between, but these are the ones that are interesting. Control copy, and then let me copy it, copy, and then paste. So I'll actually draw three of these. So then you have that. Then let me paste it one more time, and then you have that. So obviously this would be the front carbon in every situation. If I want I could make it a little orange dot to show that that's the front carbon. And then let me draw the back carbon. I should've copied and pasted this as well. So you have your back carbon in every situation. Now, if we were to rotate this character by 60 degrees-- actually if we were to rotate the back by 60 degrees, what would it look like? Well, then we would have-- this hydrogen would move up there-- so then you would have this hydrogen. Actually if we were to move it by 120 degrees, I should say. This would be 60, and then another 120 degrees. So this hydrogen would go up there. This methyl group would now be over here, and then this hydrogen would go over here. So we've just rotated to this whole thing by 120 degrees. Now, this conformation, this was called the anti-conformation. It's the most stable because the carbon-- the methyl groups, are as far away from each other as possible. This right here is called the Gauche conformation. Let me do this in a different-- and you can view this as the second most stable. At least the methyls are staggered. They're not directly behind each other. So here the methyls are as far apart from each other as possible. If you look at the ball and stick model, I actually drew it in that conformation right here. They're as far apart from each other. If you were to flip this molecule, if you were to flip it, this methyl would get closer to this methyl, and their electron clouds would start to crowd each other. So in this situation this is anti, most stable. If you rotate a little bit they'll get a little bit closer but they'll still be staggered. You get the Gauche conformation. Now, if we rotate this, if we rotate the back guy now 60 degrees clockwise, what's going to happen? Well, then you're going to have an eclipsed conformation, where the carbons are directly, but where the methyl groups are directly behind each other. And that's going to be your least stable situation. Right? So you'd have this guy-- and I'll draw it slightly-- so you'd have this guy, ch3 there, and then you would have your hydrogens that are right behind each other. So a hydrogen and a hydrogen. So in this situation where eclipsed-- this is the least stable, and also the most potential energy. And then if we were to go another 60 degrees from this, then we'd go to another Gauche conformation. If you rotate this another 60 degrees, then you'd have a ch3 here, and then you would have-- this hydrogen would be up here, and then this hydrogen here. So this is staggered. The methyl groups are, at least they're not directly behind each other, but they're not as far as they could be if we were to rotate another 120 degrees and get to the anti-conformation. So this one right here is also a Gauche conformation. So hopefully you understand now that, you just have to pick two carbons and then you can, if there's, kind of, big things attached to each of those carbons, you can just represent them as groups. And when you do that-- you'd use a Newman Projection for any part of a molecule. And when you do that, you can start to think about how it can rotate and what parts, or what versions of it, will be--