R/S isomerism occurs on a tetrahedral atom (such as carbon) when four different chemical groups are attached to the central atom. In the tetrahedral arrangement there are two different ways to arrange the four chemical groups in three-dimensional space. R-1-bromo-1-chloroethanol is loaded in the first frame. S-1-bromo-1-chloroethanol is loaded in the second frame. Both are loaded in the third frame. Use the following buttons to get back to this molecule and the starting orientations:
 

 
Make sure to notice central carbon and the four different groups attached to it. Click-drag to rotate. Shift-double-click-drag to translate. Shift-click-drag or just zoom drag (after selecting a frame) to zoom in or out. Option-click-drag to rotate around the z-axis (of the screen). One way to see that the two molecules are different (we will say non-superimposable) is to align the central carbon and two of the four groups. If you return to the starting orientation for the first two frames, you see that the methyl group, the central carbon, and the alcohol group are lined up, but in the R model the bromine is pointing toward you and the chlorine is pointing into the screen, whereas in the S model the chlorine is pointing toward you and the bromine is pointing into the screen. Try as you may (try it!), there is no way to superimpose all five atoms. If the chlorine and bromine are aligned, then the methyl and the alcohols groups are switched. Another way to see this is arrange the two molecules so that it is easy to see that they are mirror images of each other. Return to the starting orientation for the first two frames. Now rotate the S isomer in the second frame so that bromine, the chlorine, and the alchol group are pointing toward their corresponding groups in the R isomer. When you do that, you notice that the methyl groups are pointing in opposite directions. There is a mirror plane between the two molecules. Thus, these are non-superimposable, mirror images. We call them enantiomers. They are also known as chiral molecules.
 
The third frame allows you to move the two isomers relative to one another and actually superimpose (as much as possible) one molecule relative to the other. Use the button to initiate this process. This button only affects the third frame. When you click-drag one of the atoms in one of the molecules it is translated relative the other molecule. When you option-click-drag one of the atoms you rotate one of the molecules without rotating the other. To return to the normal mode of translating and rotating (both molecules) click anywhere not on one of the molecules. Try to align the molecules as best as you can. Superimpose the central atom and two of the four groups attached. Notice what happens with the other two. You can also arrange them to accentuate the mirror image idea. Orient them so that three of the four groups attached to the central atom actually touch each other. What do you notice about the third group? The following buttons do some of the superposition operations. Once you do a superposition you may have translate one molecule relative to the other so you can distinguish things clearly.
 

 
Use the following buttons to control spin, display hydrogens or not, display style, and view center.
Molecule 1 Options:
Molecule 2 Options:
Molecule 3 Options:
 
More examples of R/S stereoisomers are given below. The first of these is glyceraldehyde, the simplest carbohydrate. Load the glyceraldehyde models with the following buttons: D-glyceraldehyde has the R-configuration, while L-glyceraldehyde has the S-configuration. Later, when we talking about how to assign R vs. S (and D vs. L) come back and convince yourself of this. Chiral biological molecules like carbohydrates and amino acids use the D/L system of nomenclature, where the frame of reference is glyceraldehyde. Investigate glyceraldehyde as you did with R/S 1-bromo-1-chloroethanol. Notice the central carbon with four different groups attached. Try to superimpose the molecules. Note that the central carbon with two other groups can be superimposed but the other two groups cannot. Orient them so you can see that they are mirror images. Use the buttons from earlier to control spin, hydrogens, display style, and view center.
 

 
Amino acids commonly found in proteins are chiral (except for glycine) and follow the D/L convention similar to glyceraldehyde. The L stereoisomer is the most common. The amino acid R-group is in the place of the -CH₂OH group in glyceraldehyde, the carboxylic acid group in the place of the aldehyde, and the amino group in place of the alcohol group. Load leucine with the following buttons. Investigate its structure as before.
 
Glycine is the one amino acid that is not chiral at the alpha carbon. Load glycine with the following buttons. Notice particularly that there are not four different groups attached to the central carbon. Arrange the glycines so that one is the mirror image of the other, but then superimpose the two molecules. So glycine would have superimposable, mirror images and would not be enantiomers.
 
Here is a repeat of the buttons to control spin, display hydrogens or not, display style, and view center.
Molecule 1 Options:
Molecule 2 Options:
Molecule 3 Options:
 

 

 

Determining the Configuration in the R/S System

Reload the R/S-1-bromo-1-chlorethanol models.
The R/S configuration notation is known as the Cahn-Prelog-Ingold (CPI) system after the chemists who developed it. Each of the four groups attached to the central or chiral atom (usually a carbon) is assigned a priority based on the atom's atomic mass. In the case of 1-bromo-1-chloroethanol there are four groups attached to the 1-carbon: a bromine, a carbon, a chlorine, and an oxygen. If these are ordered from high atomic mass to low, you get Br > Cl > O > C. You view the atom from the central carbon to the lowest priority group pointing to the back. Do this for the models in the first two frames. R (the Latin "rectus" for "right") is the configuration where the other three groups pointing toward the front go from high to low priority clockwise (to the right). In this case that is Br to Cl to the O of the alcohol group. S (the Latin "sinister" for "left") is the configuration where the other three groups groups pointing toward the front go from high to low priority counter-clockwise (to the left). Use the models to convince yourself that Frame 1 displays the R configuration and Frame 2 displays the S configuration.