Determining Absolute Configuration When Group 4 is Not Back

We covered the basics of determining the absolute configuration (finding R and S) of molecules here in a previous post. That post assumed that the group 4 or lowest priority was "in the back." In order to determine the absolute configuration, we must have group 4 in the back. Also, on a side note, group 4 doesn't have to be a hydrogen although it often is in many examples. So if group 4 is not on a wedge, what are we to do?

Let's look at an example where group 4 is on the wedge, or pointed out.

Ranking priority groups using atomic number

Here, we can see that after determining the priorities of molecules, group 4 is on a wedge or coming out of the page. So if you were to try and assignme absolute configuration, just by tracing the numbers, you would be incorrect. In fact, we are trying to determine the absolute configuration looking at the molecule 180 degrees in the opposite direction.

Clockwise rotation looks like (R) configuration.

Now, we can take advantage of the last sentence; we are looking at our assignment 180 degrees in the wrong direction, or the opposite direction. This means, that all we need to do is to give the opposite configuration. So, in our example, what looks like (R) is actually (S).

The real configuration is opposite.

Now, what if group 4 is "in the plane of the page) or on the solid line? We can solve this in a couple of ways; arranging your orientation so that you are viewing the molecule with hydrogen in the back or you can apply the "single swap method."

Let's take a look at this example:

A stereocenter with group 4 in the plane of the page.

In this example, you can see after ranking each of the groups that group 4 is neither on a dash or a wedge. If you can imagine yourself viewing the carbon-hydrogen backbone, you would see something like this:

Viewing group 4 along the carbon-hydrogen backbone.

Here, we can see that if we position our eye along the carbon-hadrogen backbone, group 2 will be to the right, group 1 to the left, and group 3 is pointed down. Remember, this is similar to a Newman projection in the chiral carbon is blocking the hydrogen behind it from your view. Now, viewing correctly such that group 4 is in the back, you can simply trace around the molecule and make your assignment. However, if that seems tricky, you can use another trick, the "single swap method."

To apply the single swap method, take group 4 and the group that is on the dash and swap them out. Now, trace from 1-2-3 and so on. Note the configuration here. Now, the important part is that you have determined the absolute configuration of the swapped molecule, not the molecule you originally had. This means that the original molecule is the opposite configuration. Take a look below.

Applying the single swap method.

The single swap method works because a single swap between any two groups gives you the enantiomer of the compound that you have. All you have to do is remember that you determined the absolute configuration of the swapped version of your molecule and so the real configuration is the opposite. This little "trick" can save you valuable time on an exam.