Hello organic chemistry students! In this video we're going to talk about a new concept called Newman projections. Now this is a big fancy term for a relatively simple concept.
All a Newman projection is talking about is free rotation. about the carbon-carbon single bonds in an alkane. Now, if you imagine your right arm, or your left arm for that matter, you can go ahead and walk around every day like Frankenstein holding it in front of you, or you can hold them straight up in the air or right down next to your side.
No matter what, you're still the same person no matter where you hold your arms. Now, that is what a Newman projection is. We're going to show all the possible rotations in a carbon-carbon bond system that shows all the possible energy levels.
As you can imagine, walking around like Frankenstein all day, you're going to get tired. Straight up in the air is a little bit easier than straight up in front of you because of good old gravity. But putting them right next to your sides as you're walking around is by far the lowest energy. And we're going to see this in the molecule shown right here of butane. Now, when we look at butane, I'm going to specifically look at the C2 to C3 bond.
And I'm showing you the nomenclature down below for the numbering. Now we can rotate this sigma bond because there are no pi bonds on it. I want to stress that very, very intently right now. We can rotate sigma bonds when there are no pi bonds attached. The moment you put a pi bond down, there's no rotation between those two elements.
So if we rotate the C2 to C3 carbon bond, we can imagine as it rotates, carbon 4 gets pushed down and that's it right here. These are the same molecules, just rotating the carbon bond. If I rotate the carbon bond again, but instead of moving this carbon bond, I rotate this one going up, I form this structure right here. Now, all three of these are the same exact molecule. We're just showing their free rotation in space.
Now, we have to figure out a way of doing that organically on a two-dimensional piece of paper, even though we're talking about three dimensions. And this is where Newman projections come in. Excuse me, I had a cough. So now, I want you to imagine if I'm asking you to look down the C2 to C3 bond of butane.
Now notice this is my horrible eyeball, that's the way we like to draw in organic chemistry. And I'm looking down the C2 to C3. What we have to do is look straight down that bond, which means the C2 and C3 carbons are going to be right on top of each other. So here I'm going to draw C2, Here I'm going to draw C3. So if I try to do that, here is the C2 carbon, here is the C3 carbon, right on top of each other.
That's pretty hard to see. On that C3 carbon, we happen to have a methyl group going straight up and then two hydrogens off to the side. What about the C2?
The C2, we have a methyl group going down and two hydrogens off to the side. Now we can kind of see all the groups attacking. attached to C2 and C3, but we still can't really distinguish between these two right here.
And that's where the Newman projection concept comes up into play. I'm going to go ahead and show it, and then we'll go over the illustration of it. There is the Newman projection diagram for C2 connected to C3. Now the question is, which one is C2, which one is C3? We have a large circle, and then a small dot in the middle.
Now, if you're sitting here right now just watching this video and you look across your room, the farther away an object gets, the smaller it becomes, right? That's common sense. When has chemistry ever followed common sense? It hasn't.
So, of course, that's not going to be the same thing here. The dot represents the first carbon you're looking down and the larger circle represents the atom behind it. So, in this case right here, let me go ahead and switch my colors again. This is the C2 carbon and this is the C3 carbon in the back.
So the bigger one is the farther away carbon, the small dot is the first one. So now when we're looking down this carbon carbon bond if you're looking down this way we have a top part of the plane and a bottom part of the plane. So as we look down this plane C2 the methyl group is pointing down and that is why we show the methyl group on C2, oh wrong color right there, going straight down. And here's our methyl group C1.
Now there's two hydrogens on it and they're going to be coming off like this because this is showing our 109.5 degree bond angle between these two elements. So now we see this one carbon and three bonds being shown. But where's the fourth bond?
Oh, let's not forget that C2 is connected to C3 right back here. And that's where the fourth bond is. Let's go. Go ahead and look at C3.
C3, here's our plane once again. This methyl group is pointing straight up, so above the plane, and there's the methyl group for C4, and then we have two hydrogens on it as well. Now, this is our first diagram for the Newman projection for butane, just like if we had our hands straight down next to us. This happens to be the most stable one. What we want to do right now is show all the possible rotations of this molecule about C3.
the C2 to C3 bond. So we're going to look at the C2, C3 Newman projections. I have to specify what carbon-carbon bond we're looking down in order for you to do this. If I don't specify it, you can do any bond that you would like, but I'll always specify which bond I want you to look down.
All right, so let's go ahead and move this up a little bit. I'm going to redraw that Newman projection that I just did. Here is the C2. The C1 methyl is going straight down.
Here are the hydrogens coming up. off to the side. Methyl group on C3 pointing straight up above the plane and our two hydrogens as shown. Now what I want to do is show the rotation in this molecule and what we do is 60 degree rotations. You have to have it you're going to make a choice right now.
Which carbon do you want to hold constant meaning you don't rotate? You can hold the C2 constant or the C3 constant right here. You get to choose whichever one you would like, but you must be consistent for the rest of this problem. No changing it whatsoever.
Oh, let's go ahead and move this up a little bit. So now we can't change it once we decide. What I'm going to do is hold the first carbon constant. So I'm going to redraw my line angle formula and show the first carbon right here constant. I'm not rotating it whatsoever.
I'm going to rotate the back carbon. Now I'm going to take the back carbon and rotate it 60 degrees in the clockwise direction. You could rotate it in the counterclockwise direction as well, as long as you do it throughout the entire problem. So the big important thing here is consistency.
So here I'm going to say we're going to rotate the structure, and this means by a 60 degree rotation. So the methyl group that was pointing straight up is going to be right on top of that CH bond from C2. The carbon-hydrogen bond from C3 will be right on top of the carbon-2 methyl group. And then the hydrogen from C3 and C2 will be right on top of each other. So if you notice, we've gone from this nice, very spacey orientation to this very cumbersome rotation where bonds are right on top of each other.
Of course, we have a term for this. This first one is called the staggered conformation. So the staggered conformation means that all atoms are 60 degrees from one another.
That's wonderful. This other one is called the eclipsed conformation where we have carbon substituent bonds right on top of each other as we see right here and here and here. So of these two, which one do we think is more stable? The one replacing the electrons in these bonds farther apart from other bonds with electrons or ones that we put these electrons right on top of each other creating high electron environments?
And of course, the staggered is more stable. We're creating more space for those electrons to repel one another. So staggered is more stable than eclipsed. We're going to rotate this structure one more time. So here's my rotation by 60 degrees, but once again I'm going to hold my first carbon constant.
Nothing changes there. And now let's draw our back carbon. This methyl group rotates 60 degrees, so now it's in the... oops I was about to write 60 there.
We are now in the... Methyl or the staggered conformation. Here's the hydrogen and here's the hydrogen right here. Wonderful.
We're now staggered. Let's rotate another 60 degrees. When we rotate another 60 degrees, the first carbon remaining constant, notice I'm not showing the exact dot right there, it's implied.
Now we have the methyl groups right on top of each other and we have hydrogens right on top of each other. This is also an eclipsed conformation and the one up above is a staggered. conformation. I don't know why I'm doing staggered and eclipsed and green and red, but there we go.
So now, staggered is always more stable than eclipsed. So these two staggereds are more stable than these two eclipsed. The question I'd like to ask is, out of this staggered conformation here versus this one, which one is more stable?
Number one or number three? And the answer is number one. Why?
The biggest group on this first or the second carbon is the methyl group. The biggest group on the back carbon is the methyl group as well. So those big bulky groups that have lots of electrons that want to repulse one another are really far apart from one another, and that's very stable. So if we had to compare staggered conformation one to staggered conformation number three, this one is more stable than this one right here.
And what we call this structure all the way over there, and we use a different color right here, this is the anti-conformation. Why is it anti? The biggest groups on both carbons we're looking down are on opposite sides of one another. That's an important distinction right there.
The anti-conformation exists in staggered conformation only, and it's showing the two biggest, bulkiest groups 180 degrees from one another. Now, please forgive me if you happen to hear my cat. She's just jumped up onto my lap being quiet, but in case she starts to talk, you'll understand why.
All right, let's continue to rotate the structure. We're going to rotate it one more time. The front carbon remains constant as always because that's what I chose but you could have held the back one constant. The methyl group rotates another 60 degrees so we're now over on this side.
Here's a hydrogen and a hydrogen right here. So, this is a staggered conformation. Now, this staggered conformation has the two biggest groups right next to each other. This is what we call the eclipsed conformation. Right here.
Not the eclipse conformation, I'm so sorry, but the gauch confirmation. The gauch conformation talks about the two biggest groups being 60 degrees from one another. But wait a second.
Isn't this one also 60 degrees from one another? That sh... Sure is.
So this is also the Gauss conformation. So we can have multiple Gauss conformations in the same structure. We're going to have one anti as shown.
Now, continuing our rotation again. The first carbon remains constant. Wonderful.
Here it is. It looks like it gets sloppier as I draw it. I so apologize.
The methyl group is now right on top of that hydrogen, hydrogen, and hydrogen right here. That's an eclipsed conformation. And then we rotate one more time and we are right back to where we started.
We have now just drawn all six Newman conformations for butane looking down the C2 to C3 bond. You will always have six conformations. If some of them happen to be equal to each other, that's okay.
But as a whole, we have six conformations. So now what I'd like to do right here in blue is call this confirmation one. confirmation two, confirmation three, four, five, and you got it, six right here. Now in addition, I'm going to shrink this down a little bit without trying to ruin the structures. There it is.
And what I want to do over on the right hand side that I just made is give the energy diagram for these confirmations, relative energy diagrams. We're not going to put in specific values. I'm not going to pull up the data chart for that. We're just going to do relative to one another.
So I'm going to go ahead and put compound 1 right here, just relative. Now, compound 1 versus compound 2. Is compound 2 more or less stable than 1? It is more unstable because this is the Eclipse confirmation.
Eclipse is always more unstable than staggered, always. So I'm going to go ahead and put 2 right here. The first two are the easiest. After this, it gets a little trickier.
We're now in staggered. We know staggered is going to be lower in energy than this eclipsed. But the question is, is this staggered going to be higher or lower in energy than this one that we've turned anti?
Compound 3 right here, or confirmation 3, is going to be higher in energy because the two methyl groups are near to one another. So I'm going to go ahead and put 3 right here. It doesn't matter exactly how far up or how far down you put it as long as you're showing it relative to 1. that it's higher in energy.
That's the important thing. Now, let's go ahead and go to four. Four is eclipsed, the same as two.
We have to ask ourselves, though, out of two and four, which one is higher in energy, aka the most unstable? We have a hydrogen on methyl, hydrogen on methyl, hydrogen on hydrogen. Well, that's not bad. Hydrogen's pretty small.
Over here, hydrogen on hydrogen. That sounds great. Hydrogen on hydrogen.
This is living the dream. Methyl on methyl. The two biggest groups right on top of one another. Massive electron repulsion making that the most unstable conformation. So here when we have the two biggest groups right on top of each other, that makes the highest energy compound.
Now, gauch is going to be lower than eclipsed. But here, isn't this gauched 5 the same as this 3 right here? Methyl next to methyl.
Oh, methyl next to methyl. Four hydrogens. Four hydrogens. Two and four, or sorry, two and three, oh my gosh, five and three right here happen to be of the same exact energy.
So we get to put five at the same energy, and let me get the same color in there. Five as the same energy, and I'm just making mistake after mistake here. Let me get rid of that. There it is.
Five as the same energy as three. Identical. Let's go ahead and look at 6. 6 right here. Does any of the eclipse match this? 6 versus 4. They're different, aren't they?
Here there's a hydrogen on methyl, hydrogen on methyl, two methyls. Strikingly different. 2 and 6 though, aren't they the same? They sure are.
Which means 6 is going to be of this energy level right here. And we've just shown the energy diagram for butane. Now, I highly recommend that you redraw butane and do this Newman projection again. In the discussion section for this class, we will go over more complex problems. What you have to consider here is the relative size of the substituents on the Newman projection.
So let me go ahead and just move this up for a second. If I was to give you a structure like this, and I'm going to put an isopropyl group here and an ethyl group here. Now I want to look down the C2 to C3 bond of this molecule, which means we're looking right down this plane right here. If we assume the isopropyl is a substituent, not part of the main parent chain.
So I'm going to go ahead and indicate this bond right here. So when we draw this on this first carbon, I have a methyl group going straight down, an isopropyl group coming over here, and a hydrogen right there. Why couldn't the isopropyl group be on the other side? For this class, it sure can.
If this was the major's organic chemistry, it would not be on one side. It would only be on the one side because of stereochemistry. For this class, we can put it on either one. We're perfectly fine.
Now let's go and look at the back carbon. Back carbon, we have an ethyl group sticking straight up. I'm going to abbreviate ET. Here's an ET for the other ethyl and a hydrogen. Wait a second.
What are the biggest, bulkiest groups on each carbon? On the first carbon, the C2, the isopropyl is the biggest group right there. Let's look at the back carbon for a moment.
There are two ethyl groups, which means this is the biggest. This is the biggest. Wow. That means we're going to have multiple Gauss conformations as well as two. anti-conformations because we can have this ethyl group here or this ethyl group here, two different anti-conformations.
Now, how do we know that the ethyl group is bigger than methyl? It has more carbon in it. How do we know isopropyl groups are bigger than ethyls?
More carbons taking up more electron space. So the bigger the group, the more space it takes up, more electron repulsion, and it dominates the ring. That's very key and critical here.
Now if I was starting to add in other elements such as chlorine and bromine in the question, I would state that, oh, a methyl group is bigger than a chlorine. Okay, so if a methyl group is bigger than a chlorine, an ethyl group is definitely bigger than a chlorine, and an isopropyl group is by far bigger than chlorine. So I would state those odd ones, but the carbon chains I will expect you to be able to rationalize, the bigger and more bulkier that they are.
the more bulk that they have and more electron repulsion, and they dominate that position on the carbon in question. So with that right there, this covers the basic comments of, or the basic ideas of, Newman projections. We'll talk more about this in the discussion section, kind of go over example problems of it, but this is how we solve them. So if you want to come to the discussion section, that would be wonderful, but if not, please make sure you watch the video over it for the session that we have.
and you can learn more about Newman projections. I hope each of you are doing well.