Cahn-Ingold-Prelog Rules

These are rules that allow for the universal description of the geometries (and configurations) about stereocentres.

The first thing to do, is to assign priorities:

To do this, we simply place the substituents on a stereocentre in order of their atomic numbers (starting with the highest first). To distinguish between different isotopes, rank the heaviest as higher priority.

In cases where there are two substituents of the same mass, then the masses of their nearest neighbours are compared, until they can be distinguished. For example:

Chlorine has the highest atomic number, and therefore has highest priority.
Second comes the acid group, because although both it and the methyl group have carbon atoms attached to the stereocentre, the carbon of the methyl only has hydrogen atoms attached, whereas the carbon atom of the acid group has oxygen atoms attached. As oxygen is heavier than hydrogen, the acid group has second priority, followed by the methyl, and lastly, the hydrogen atom.

Now we have established the order of priority, we can set about determining the absolute configuration.  This is done by imagining that we are rotating the molecule in question so the lowest priority group (hydrogen in our case) points away from us.  Now imagine looking along the line formed between the stereocentre and the hydrogen.  It should look something like this:

Now we simply follow the direction of decreasing priority.  That is, count from 1 to 3.  In doing so, we notice that the groups are anticlockwise, and this is designated as an ‘S’ configuration.  Had we chosen the other enantiomer of the molecule above, the situation would have been:

Which, viewed as before, looks like:

This time, the groups decrease in priority in a clockwise manner, and this is designated as an R‘ configuration.