R and S configuration on Newman projections - Chemistry Steps (2024)

To determine the R and S configuration of a chiral carbon in Newman projections, we need to look through the bond such that the lowest priority group is pointing back. But first, one question – why would anyone need to determine R and S on a Newman projection, right?

The need for this is relevant mainly when a molecule is drawn such that the regular strategy for determining the absolute configuration is not so straightforward.

For example, what is the configuration of the following molecule?

The problem here is that the lowest priority group is neither pointing away or towards the viewer (wedge/dash). This means we cannot determine the configuration by simply following the Cahn-Ingold-Prelog rules. So what we can do is draw theNewman projectionof the molecule looking from the angle that places group 4 in the back (pointing away from the viewer).

The lowest priority group is pointing back and therefore, the clockwise direction of the arrow indicates an R configuration.

Now, what if the Newman projection represents a compound with two chiral centers?

For example, determine the absolute configuration of all the chiral centers in the following molecule:

In this case, converting the Newman projection into a bond-line structure might be a better way of doing this. There is a detailed post with a lot of visual elements about converting Newman and Fischer projections to a Bond-Line Structures, so check that out before doing the R and S part but this is how you can do it.

In order to convert a Newman projection to the corresponding bond-line structure, you need to look at it from the side. So, first, decide the direction you are going to use – it can be any – right or left unless specified in the question.

For example, we can look at this molecule from the right side which places the Cl on the bottom-right corner and the methyl on the top-left in the zig-zag form:

After this, add the groups on the corresponding carbons pointing them towards you (wedge) and away from you (dash):

If this is confusing, it might be helpful to convert the Newman projection to Haworth before getting the final structure in the bond-line. Haworth projection is different from the Newman in that it shows the bond between the front and back carbons. So, it is not looking directly through the bond, but rather at a slightly tilted angle:

After this, we can now project the Haworth into bond-line and place the groups according to their arrangement:

Now, we can determine the configuration of the two chiral centers. Let’s start from the carbon atom on the right. The highest priority if the bromine, followed by the chlorine and the carbon atom on the left since it is connected to an oxygen. Finally, the methyl group is the lowest priory:

The arrow goes clockwise and with the lowest priority in the back, it is an R configuration.

For the carbon on the left, we have the oxygen as the highest priority, followed by the carbon atom on the right since it is connected to Cl and Br, after this, the carbon of the ethyl group is the third, and the methyl group is the lowest priory:

However, the priority 4 is in the plane of the molecule, and this is when drawing a Newman projection looking through the corresponding C-C bond may become handy. We will do it such that the methyl group is in the back and therefore, the direction of the arrow will tell us the absolute configuration:

The wedge and dash (configuration) of the other carbon with the Cl and Br does not matter for this purpose as the priority only depends on the atomic numbers.

The counterclockwise direction indicates that this carbon has an S configuration.

Overall, these are the configurations:

If drawing a Newman projection does not look like an option you’d take, remember that there is also the swap method for determining the R and S configuration.

So, when the lowest priority is neither wedge nor a dash, there is this simple yet such a useful trick making life a lot easier. And that is the swap method. The basis of this is that swapping any two groups on a chiral center inverts (changes) its absolute configuration (RtoS,StoR):

Notice that these are different molecules. We are not talking about rotating about an axis or a single bond, in which case the absolute configuration(s) must stay the same. We are actually converting to a different molecule by swapping the groups.

And we can use this to make it easier determining theRandSconfiguration for any molecule regardless of how it is drawn.

For example, in the last molecule, after determining the priorities, we can swap the methyl group with the OH thus making it a dash. After this, whatever we get for the configuration based on the arrow, we change it since the groups had been swapped:

The arrow goes clockwise, however, because the groups had been moved, the absolute configuration is S.

Likewise, for the first molecule, thelowest priority group was in the plane of the molecule, so what we could do is swap it with the one that is pointing away from us (Br). After determining theRandSwe switch the result since swapping means changing the absolute configuration and we need to switch back again.

The arrow goes counterclockwise indicatingSconfiguration and this means in the original molecule it isR.

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