Practice: Draw the most energetic Newman Projection of CH3CH(C6H5)CH3
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As we learned already, we use Newman projections to visualize the rotations of conformers. Now we will learn the steps involved to draw the perfect one.
Worked Example: Draw the most energetically favorable Newman Projection for CH3CH2CH2CH2CH3 down the C2 – C3 bond.
1. Convert problem into bondline structure
Concept #1: Step 1
So the first thing that I always do is if you're given a condensed structure, which is often the case, you need to convert the problem into a bond line structure. What that means is that I want to take this five-carbon chain or whatever I'm given and turn it into bond line. So that's the first thing I'm going to do. Five carbons right there, so this is pentane.
2. Highlight the bond of interest
Concept #2: Step 2
The second thing I'm going to do is I'm going to highlight the bond of interest. What is the bond of interest? What? It's this, C2-C3. That's your professor telling you that he wants you to focus on a certain bond that's going to rotate.
Just like when I was talking to you about conformers, that you could have sigma with s-cis or s-trans, he's picking out which sigma you're going to use, which sigma bond you're going to rotate and that's going to be this sigma bond right there because basically what you want to do is you want to go from the second carbon to the third carbon. That's what C2-C3 means, C2-C3.
Now, it could have also been, just letting you know, it could have also been this one because if you were counting your 1 from over here, then this would have been your 2 and your 3. But I'll just go ahead and use this other one. This is my 2. This is my 3. Perfect. So I highlighted the bond of interest. You don't need to necessarily write the numbers as long as you just know what bond it is.
3. Draw an eyeball glaring down the length of the bond
Concept #3: Step 3
Then what we want to do is – this part sounds silly. I'm going to redraw this. But I actually want you to do the eyeball thing. I want you to draw an eyeball looking down the length of that bond. I want you to draw an eyeball and make it look straight at that carbon, so pretend that's you squinting your eyes at it. And you're going to try to figure out what is this thing going to look like if I was looking straight at it.
4. Surround only the bond of interest with ALL implied hydrogens
Concept #4: Step 4
Now the way you're going to do that is that you surround only the bond of interest with all implied hydrogens. That means if there's any implied hydrogens on that carbon or on that carbon, I need to add them.
How about the hydrogens on that carbon, do I add those as well? No, because that's not the bond of interest. The bond of interest is only going to be from two to three. What that means is that I'm going to add two H's here and I'm also going to add two H's here. But I'm not going to add H's anywhere else because that's not the bond of interest.
5. Draw a front carbon with 3 groups in the front and a back carbon with 3 groups in the back
Concept #5: Step 5
Now what we're going to do is we're going to draw a front carbon with three groups on the front and then a back carbon with three groups on the back like I was doing when I told you guys about the way Newman projection works.
So I'm going to say that for example, the front one is my red carbon and the back one is my blue carbon. So my red carbon I would just draw with a little dot. And I would draw that it has three things coming off of it. You can draw your little triangle thing, whatever that's called, however you want. You can start with it with a point up or you can start with a point down. It doesn't matter as long as the other one is consistent.
Basically, what I would say is then what are the three things that that red carbon is attached to. Well, it seems to be attached to an H on the top, an H on the top and then a CH3 at the bottom. That is this CH3 right here. Get that?
Then I look back at the blue one. The blue one, imagine that it's kind of peeking out from behind the red one, so the blue one is going to be a circle behind and then I'm going to draw the three groups that the blue one is on, that the blue one has
So the blue one seems to have two H's, H and H. And then what else does it have? Well, it has a two-carbon chain coming off of it. So that would be what you could just write as CH2, CH3. Does that make sense? Another way to write that would have been to write Et, which stands for ethyl.
Another way to write CH3 would be to write Me, which stands for methyl. There's abbreviations for a bunch of these different ones. And your professor might use those more than he uses the actual letters.
6. Determine which dihedral angle would correspond
Concept #6: Step 6
So now that we've drawn that Newman projection, that is a valid Newman projection. That could be right. The only thing is that I don't know if it's the energy state that the professor was asking for because the professor could ask for any energy state. He could ask for anti. He could ask for gauche. He could ask for eclipse. Maybe even something in the middle. So I have to make sure that this is the exact one that he wants.
Then determine which dihedral angle would correspond. I have to go up here and see what he said. Well, he specifically said draw the most energetically favorable. What does energetically favorable mean? Well, I'll just tell you right now, if you see something that says favorable, that's a good thing. That means stable. So we're looking for the most stable conformation.
What is the most stable conformation? That's going to be anti. Remember anti is the most stable. Let's go down and see if that's what I drew. And what's the bond angle, the dihedral angle, by the way, for anti? 180. Let's go down and see if that's what I drew.
What I have is a large group in the back and a large group in the front. They appear to be 180 degrees away from each other. So this would be anti. So this would be your right answer. This would be what would get you the points on the exam.
So even if I drew it wrong, let's say I drew the wrong conformation at the beginning, you could still rotate it into the right conformation. The important part is that you're following all these steps.
Hint: This question asked for the most energetically favorable = most stable. Which conformation is most stable?
The right answer was anti. You got it. So it turns out this time we drew it correctly on the first try. But there will be other examples where we will have to rotate the Newman Projection into the correct position.
Practice: Draw the most energetic Newman Projection of CH3CH(C6H5)CH3
Hint: Not all Newman Projections can form an anti, gauche and eclipsed conformation. If you have no clear large group on one side of the projection, you’ll just be stuck with projections called staggered (not overlapping) and eclipsed (overlapping).
Practice: Draw the most stable Newman Projection of CH3CH2 CH2OH through the C2 – C1 bond.
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