Newman projections are head-on representations of molecules looking down the bonds between two carbons typically used to visualize rotation around a single bond. We refer to these different rotations of Newman Projections as conformations.
They portray the stereochemistry of substituents on two carbons, with a large circle representing the back (or distal) carbon and a small dot representing the front (or proximal) carbon.
Where bondline (aka skeletal structure) looks at a molecule perpendicular to its bonds, a Newman projection looks at a molecule down the bond between two atoms. From left to right, we’re looking at ethane as it gets rotated slightly. We start from bondline, rotate the molecule so that the carbon on the left shifts closer to us to view as sawhorse, and then we continue rotating it to look at the molecule head-on.
Let’s draw a Newman together. First things first, we need to pick out the template we’ll use: the “Y” or the “peace sign.”
It’s not so hard to determine which one to use; all we need to do is look at the orientation of the substituents on the atom closest to our eyeball. Let’s make one with good, ol’ ethane.
Okay, so I’ve gone ahead and drawn an eyeball to show exactly where we’re looking. The carbon closer to the eyeball has three hydrogens—one in plane facing straight down and the two others facing up on wedge and dash. Looking at the the templates, which one has a substituent on the front carbon facing straight down? The “Y!” Let’s add our color-coded hydrogens to the template!
The brown and yellow hydrogens are fairly easy because they’re in plane and facing straight down and up, respectively. Let’s walk through the other substituents. Remember that the big circle represents the back carbon, and the place where all the lines connect is the front carbon. Wedge means it’s coming out of the page (toward you), and dash means it’s going into the page (away from you).
In bondline, we can see that the light blue hydrogen would be to the left of the eyeball and above the carbon-carbon bond so it must be on the left in our Newman. The red carbon is on the back carbon, facing down, and to the right of the eyeball so it must be on the back carbon on the right.
Okay, now let’s get some more practice and try butane! What template should we use for this molecule from the eyeball’s perspective? I’ve drawn it looking down the C2-C3 bond.
Notice here that the carbon closest to the eyeball has a yellow methyl group (CH3) facing straight up in plane and two hydrogens facing down and on wedge and dash. What template should we use? The peace sign, of course! Let’s go ahead and add the substituents.
The methyls were pretty easy because they were facing straight up and down on different carbons, but the hydrogens might’ve been a bit tricky. This time, since we’re looking from the right of the molecule, the wedged substituents would be on the left of our Newman projection.
Wait a second… Don’t single bonds freely rotate? Yes! This is where we have to consider conformations. Looking at butane, let’s keep the front carbon (C2) exactly the same and rotate the back carbon (C3). It’s always better to keep one static and rotate the other; rotating both at the same time can get confusing.
In both Newman projections, our front carbon’s and back carbon’s substituents are 60 degrees apart from each other. We refer to this as a staggered conformation. There is a slight difference between the first Newman and the second, though.
In the first Newman, the front and back carbons’ methyl groups are as far apart as possible: 180 degrees. When the largest substituents are 180 degrees apart, we refer to that confirmation as anti, a specific type of staggered confirmation.
In the second Newman, the methyl groups are only 60 degrees apart. When the largest substituents are 60º or 120º apart, we refer to the conformation as gauche. What happens when the front and back carbons’ substituents are 0º apart and they overlap? Let’s take a look below:
In both Newman projections, the groups are overlapping. When substituents line up like that, the molecule has a higher energy (lower stability) because of the steric interactions between the front and back carbons’ substituents. This kind of conformation is called eclipsed. Just like before, there is a slight difference between the two Newman projections here.
The first Newman has two hydrogens overlapping and two pairs of a methyl overlapping with a hydrogen. Since our two largest groups, the methyls in this case, aren’t overlapping we’d call this conformation eclipsed.
The second Newman actually does have the two largest groups overlapping, so we’d call that totally eclipsed. This form of eclipsed conformation is actually the highest in energy (lowest stability) because larger groups have larger steric interactions; put simply, they bump into each other more.
Let’s put all of that together and look at a visual representation of the different energy levels of our conformations using hexane as an example. First, let’s draw hexane in its highest-energy conformer looking down the C3-C4 bond.
That wasn’t so hard, right? Notice that I actually drew two equivalent projections, but the second one uses “Et” for ethyl instead of CH2CH3. Let’s take a look at the energy levels as the dihedral angle (the angle between the front and back carbons’ substituents) changes.
All the way to the left of the diagram, we’ve got a totally eclipsed hexane with the highest energy of any of the conformations. As we rotate to have a dihedral of 60º, we drop in energy since we end up in a gauche conformation. 120º is another eclipsed conformation, so it jumps up again. Rotating to 180º gives us the lowest energy, and that makes sense because our two largest groups are anti. 240 degrees is the same as 120 degrees, 300 is the same as 60, and 360 is the same as our starting point.
Cyclohexane chair conformations can also be portrayed through a Newman projection, but it’s a little bit different. We actually use what amounts to two Newman projections stuck together, and we call it a double Newman. Let’s draw one for (1R,2R,4S)-4-chloro-2-iodo-1-methylcyclohexane.
Basically, all we have to do is create two separate Newman projections and link them together through two different carbons. It really helps to choose two carbons that don’t have substituents for this. I’ve color-coded the different carbons here to help keep track.
Notice that my double Newman can be split into two different Newman projections with the “Y” template. That’s because, on the chair conformation, the iodine is axial down and the chlorine is equatorial up. Let’s convert the chair conformation to planar and see if it’s a bit easier to see what that would look like.
Converting the planar cyclohexane to Newman is a bit easier, right? See how I’ve drawn two extra eyeballs? The red eyeball is looking down the bond between the chlorine’s carbon and the red carbon, and that bond’s Newman projection is on the left of the double Newman. The blue eyeball is looking down the bond between iodine’s carbon and the blue carbon in the back, and that bond’s Newman projection is on the right.