Conformations of Newman Projections

We use Newman projections to visualize the rotations of conformers. Some are more stable than others. 

Concept: How sigma bond rotation is visualized

4m
Video Transcript

Now that we understand that sigma bonds are free to rotate as much as they want, it turns out that there's a unique way to visualize this rotation and that visualization technique is called the Newman projection. Newman projects are all about finding the different energy levels that conformers can make by rotating a sigma bond.
So maybe you recognize this drawing. This drawing is the same drawing that I had before when I was talking about conformers. And I was saying that you could have an s-trans hexane or an s-cis hexane. But it turns out that it's kind of difficult to visualize how those are different in energy. Energy is kind of an abstract concept right now, we haven't really defined it really well. But just think about that if something is very high in energy, that's not going to make it very stable.
In this case, it's not easy to tell which one is lower energy and which one is higher energy and that's why we want to have a new way to visualize this. What we say is, hey, imagine that your eyeball was right on this plane, so imagine that this is your face and this is your nose and those are your lips and obviously you're just an amazing looking dude. Wow. I need to stop what I'm doing because that's really terrible looking.
Imagine that that's a person. Don't imagine that's you, I don't want you to get traumatized. And you're looking at that bond straight down the line, straight down that bond. What you would actually see is you would see a carbon in the front, let's say that's your red carbon. And you would also see a carbon in the back, let's say that's your blue carbon. You'd see them overlapping each other. If you're looking right down that bond, you'd see one in the front, one in the back. So you would see that this would be your front one and then the one in the back would be represented by that circle.
Basically, what Newman projections allow you to do is to visualize where are the groups on the front carbon oriented and where are the groups in the back carbon oriented and how are they related to each other. With Newman projections, you're allowed to rotate around that bond in a three-dimensional way.
Basically, if you were to look down that bond what you would see is that your big groups, this ethyl group here and this ethyl group here would be on opposite sides of these carbons because as you can tell, if you were to draw your dotted line, they're on opposite sides of the fence. Basically, they'd be really far away from each other.
But then if you look at s-cis, s-cis is different. S-cis, if I had the same thing where my eyeball is looking, I would still see one in the front – oh, just a second. Wow. Red. I'd still see one in the front and one in the back, but what I'd also see is that now the big groups are overlapping each other. Instead of being on opposite sides, they're overlapping and that has a huge difference in how stable these molecules are, how stable these conformations are.

So Newman projections are way of analyzing which sigma bond rotation will be the most stable. We have some rules about which rotations are better and which are worse!

Three Types of Conformations

The dihedral angle (theta), is equal to the angle between the two largest groups on either side of the projection.

Concept: The energy states of 3 different Newman Projections.

4m
Video Transcript

So let's go ahead and start off by defining what the dihedral angle is. The dihedral angle is defined by theta. Theta is just a variable that means angle. What it does is it describes the rotation between the two largest groups relative to each other. So basically the largest group in the front, on that front carbon, that largest group on the back carbon, where they are relative to each other, that's going to tell you your dihedral angle.
For example, if my two largest groups are overlapping each other perfectly like that – by the way, I just want to point out that there is a mistake on mine how it says CH3, it should be CH2 and then CH3, so don't let that freak you out. If they're perfectly overlapping, that means that the difference in their angle is zero because as you go further around, your angle gets bigger, but they're perfectly overlapping. So the dihedral angle, in this case, would be zero. And this conformation is called eclipsed.
The eclipsed conformation is the one where both of the groups overlap each other perfectly. This happens to be the conformation with the highest energy. Why would that be? Well, it turns out that large, bulky groups don't like to be next to each other. Why? Because they feel crowded. Remember that big bulky groups, they don't just have atoms, they also have electrons and electrons are negative. So if you put a bunch of these electrons together, they're going to repel each other.
So it's a really bad idea for these groups to overlap each other so perfectly. So this is going to be highest energy and what that means is lowest stability. Energy and stability are inverse of each other. They're opposites.
Then let's look at the next one. The next one would be if the dihedral angle is 60 degrees, 60 degrees just means that they are not perfectly overlapping, but they're also not perfectly far away. They're just kind of like maybe 2:00. At a 60 degree angle that is going to be called a weird word. That's going to be called gauche. Gauche, you can say it's when they're adjacent to each other. They're not overlapping, but they're still not as far away as possible, so in this case you see how its visualized there.
Now this one is not amazing, but it's not as bad as the other one. What I'm trying to say here is that the gauche conformation it's not as unstable as eclipsed, but it's also not as good as it could be. I'm just going to say here it has middle energy. That's a terrible word to use. But middle meaning just that it's not as bad as eclipsed so that would also mean kind of middle stability. Cool.
So now we're going to go on to our last one. Our last one is what if the dihedral angle is 180 degrees. That means that they're perfectly apart from each other. If they're perfectly apart from each other that means they're the furthest away they can get. And that means this is going to be called anti or anti. The anti conformation.
The anti conformation is where your two largest groups are opposite, that one's going to be the lowest energy. And by lowest energy that would obviously mean most stable. 

Plotting a Newman Energy Diagram

As you’ll see, when we plot (theta) against energy, we wind up getting a predictable pattern of peaks and valleys that can be used to better understand the different rotations. 

Concept: How to draw a Newman Projection Energy Diagram.

8m

Professors may ask you to draw this, so don’t just tune it out! You need to understand the basics of energy diagrams for this topic. 

Conformations of Newman Projections Additional Practice Problems

Draw the three lowest energy conformations of 3-methylpentane sighting down the C2-C3 bond in order of increasing energy (i.e. most to least stable).

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Which is 2-chloro-3-methylbutane?

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Conformational analysis is the study of the energy changes (stability) of different spatial arrangements of atoms relative to rotations about single bonds. Perform the following analyses:

Using Newman Projections, fill in the potential energy diagram for the rotation around the C2-C3 bond in butane (i.e. draw each Newman Projection onto the diagram at each angle of rotation specified). ]

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Sketch an energy diagram that shows a conformational analysis of 2,2-dimethylpropane. Does its energy diagram resemble the one for ethane or butane? 

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Draw the most stable Newman projection for the following compound

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Draw the most stable Newman projection for each of the following compounds.

a) 3-methylpentane (Looking down the C2-C3 bond)

 

 

 

 

 

b) 2,3-dimethylbutane (Looking down the C2-C3 bond)

 

 

 

 

 

 

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A. Sketch the curve showing the energy changes that arise from rotation about the C2-C3 bond of 2,2-dimethylpropane.

B. Label the axes of your graph.

C. Draw and label (i.e., anti, staggered, eclipsed, or gauche) the Newman projections for each conformation.

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Draw all conformations, using Newman projections, of 1,1,1-tribromo-2,2,2-trichloroethane. Label each using labels such as staggered, eclipsed, anti, or gauche.

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True or false? The specific conformation of 2,3-dimethylbutane shown below is the most stable (lowest energy) conformation possible for this molecule.

A. True          

B. False

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For the pair of molecules drawn below, choose the letter that corresponds to the  MORE STABLE molecule.

Does the UNSTABLE molecule chosen below have ANGLE STRAIN?

Does the UNSTABLE molecule chosen below have TORSIONAL STRAIN?

Does the UNSTABLE molecule chosen below have STERIC STRAIN?

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Rank the following molecule in order of increasing stability (least stable to most stable)

A) 4, 1, 2, 3

B) 1, 2, 3, 4

C) 2, 3, 1, 4

D) 3, 2, 4, 1

E) 3, 2, 1, 4

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For the pair of molecules drawn below, choose the letter that corresponds to the MORE STABLE molecule.

Does the UNSTABLE molecule chosen have ANGLE STRAIN? (YES or NO)

Does the UNSTABLE molecule chosen have TORSIONAL STRAIN? (YES or NO)

Does the UNSTABLE molecule chosen have STERIC STRAIN? (YES or NO)

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The following chart describes the relationship between potential energy and torsional angle in butane (the torsional angle being measured about the C2-C3 bond). Identify the labeled maxima or minima as representing fully eclipsed, gauche or anti conformer energies. NOTE: "anti" or "trans" have the same definition: the groups are away from each other at a 180° angle.

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The eclipsed and staggered forms of butane are said to differ in:

     (1) molecular formula

     (2) configuration

     (3) conformation 

     (4) constitution

     (5) structure

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