So far, all the questions have asked you to assign (R/S) on molecules drawn as bond-line diagrams, such as the molecule shown bottom left.
But every once in awhile, you might find yourself thrown for a loop. For example, how do you determine R/S when the molecule is drawn as a Newman? (bottom right)
The trick is to convert the Newman projection to the bond-line diagram and then assign R/S.
This post explains how to do that.
A Brief Refresher: Cats
One common thing I hear from students about why organic chemistry is hard is that they say they have “a hard time visualizing things in 3D”.
I actually don’t think this is true.
I think most people are fine visualizing things in 3D.
The problem is that visualizing molecules is unfamiliar.
Given this hypothesis, let’s take something that is familiar and do some visualization exercises.
Here’s a picture of a Jerusalem street cat.
Could you visualize what it would look like from the side?
Almost certainly, because you are very familiar with how cats look from most angles.
If you had to make a drawing (stick figures are fine) it would probably look something like this:
Note we took some liberties. The legs facing us are drawn as wedges and the ones pointing away are dashes.
[Here, I drew the two wedges on the “inside” relative to the dashes, but drawing them on the outside (or even alternating) is OK, since it amounts to the same thing]
Now let’s do the same kind of exercise, but in reverse.
Let’s take that the stick figure we just drew and try to picture what it would look like from the front (i.e. look from the left) and from the back (look from the right).
For reasons that will soon become apparent, we’ll add a bit of detail: let’s give the cat some colored “socks” (orange and blue).
[You might ask: what’s that weird looking symbol? It’s
the Side-Eye of the Illuminanti, the symbol of the underground secret society of chemists that rules the world just a symbol that says, “imagine looking at this thing from this direction”]
Because you likely have a very good 3-D mental model of a cat, you shouldn’t have found exercise this too hard.
Hopefully you got something like this, below. For simplicity, I omitted drawing in the eyes [2 in the front view, 1 in the back view (heh)]
The circle represents the cat’s body, since the front and back hips block each other.
Maybe you noticed this helpful correspondence:
- When we looked at the cat from the left (i.e. front view) the groups on wedges (orange) ended up on the right side.
- When we looked at the cat from the right (i.e. back view) the groups on wedges (orange) ended up on the left side
The Newman Projection: Eclipsed and Staggered Conformations
Of course this has all just been a roundabout way of reviewing the Newman projection, as well as an exercise in trying to help you realize that you are better at visualizing molecules in 3-D than you previously may have thought.
It helps that cats map on to molecules pretty well!
Recall that Newman projections are a convenient way of showing conformations in molecules. For example, the cat we just drew was in the “eclipsed” conformation, where the head and tail both line up with each other like the hour and minute hands on a clock striking midnight. The front and back legs line up as well.
The other significant conformation of note is the “staggered” conformation, where the front three groups are offset by 60 degrees with respect to the back three groups.
[Despite several attempts, I was unable to obtain a good photo of a Jerusalem street cat in a staggered conformation. They really don’t like being twisted. So we’ll have to work with models.]
In the example below, we’ll rotate the back carbon 60 degrees clockwise (CW) with respect to the front carbon, along the central carbon-carbon bond. After this is done, note how the green hydrogens have moved from 12:00 to 2:00, 4:00 to 6:00, and 8:00 to 10:00 respectively.
When we look at this “staggered” molecule from the side, we obtain a bond-line diagram where the bonds in the plane of the page have a zig-zag configuration (bottom right).
If we look at this “staggered” bond-line diagram from the left, we obtain the “staggered” Newman, drawn top right.
How To Convert A Newman Projection To A Line Diagram
So how do we convert a Newman diagram to a bond-line diagram? This section will walk through all the steps.
The first thing to recognize is that in bond-line diagrams there are only 4 possible patterns that the bonds in the plane of the page will follow.
There are two possible “zig-zag” shapes, corresponding to the “staggered” conformation, and there are also two possible “C-shapes” corresponding to the “eclipsed” conformation. [Note that line diagrams are often tilted 30° from these directions, but for simplicity we’re going to keep the central C-C bond strictly horizontal].
If we look from the left on each of those 4 line diagram patterns, we can see that each one generates a different Newman projection pattern.
There are 4 Newman projection patterns:
- front down/back up,
- front up/back down,
- front up/back up,
- and front down/back down.
Now that we’ve seen how the patterns work in the forward direction, let’s now apply these patterns in the reverse direction.
Using these templates, we can take any Newman projection and work backwards to get the corresponding bond-line template, and then draw in the dashes and wedges.
One important thing to note. As we saw with the cat, when we look from the left side of the molecule:
- all groups on the right (R) become wedges, and
- all groups on the left (L) become dashes
If you follow through with the pattern of looking at the molecule from the left perspective, then all you need to remember is to draw the wedges on the right side of the Newman diagram.
Three Worked Examples
Let’s apply this to a few specific examples.
First, let’s assign R/S to a Newman drawn in a staggered conformation with a single stereocenter. This one is drawn as (front up, back down).
In this example we drew the (front up, back down) staggered template, and then filled in the bonds. Note that the groups on the right of the Newman (Br and CH3) became attached to wedges in the line diagram.
You should obtain (R) as the configuration.
Next, let’s go back and do our original example (2-bromo-3,4-dimethyl pentane). It is also drawn in the staggered conformation (front down, back up).
Using the same method, you should obtain (R) for the stereocenter containing Br and (S) for the stereocenter on carbon #3. For details on how this was done, here’s the image.
What if the molecule is in an eclipsed conformation? Try this one. This follows the (front down, back down) pattern.
You should obtain (3R, 4R). To see details of how it was done, click here.
If you can visualize what a cat would look like from the front and from the side, then you should be able to convert a Newman projection to a line diagram. This is the first step in determining R/S on a Newman projection.
Knowing that there are only a few templates makes it easier.
Once you do it enough times, you won’t even need the templates, and you might find that it’s easier to just do it in your head.
Comments or questions? Please ask!
In the next post, we’ll look at the Fischer projection.
Thanks again to Matt for helping with this post. Hire Matt as your tutor!