Isomers of this type, structures in which the atomic connections are the same, but their spatial orientation is different, are called stereoisomers. Note that you can arrange the two stereoisomers in Figure 1 so that they occupy mirror image orientations to each other. While these structures are mirror images, they are not superimposable mirror images. This means that they are distinct entities. Non-superimposable mirror images are called enantiomers. When the four groups attached to a tetrahedral atom are different, that atom is said to be chiral.
In order to differentiate one stereoisomer of bromochlorofluoromethane from the other, chemists developed a convention known as the Cahn-Ingold-Prelog rules. Here's how it works.
1. Assign each group a priority from 1-4 based on its atomic number. In the case of CHBrClF, Br is assigned priority 1 because it has the highest atomic number; Cl is 2, F is 3, and H is given priority 4.
2. Orient the structure so that you are viewing along the axis from the chiral atom to the group with priority 4.
3. Trace a path from group 1 to 2 to 3. If that path follows a clock-wise direction, the stereoisomer is designated R. If if follows a counterclock-wise direction, it is designated S.
Thus the enantiomer shown on the left of Figure 1 is called (S)-bromochlorofluoromethane, while the one on the right is called (R)-bromochlorofluoromethane.
Since stereochemisty plays such a vital role in organic chemical reactions, it is essential to be able to represent the 3-dimensional structure of a chiral atom in two dimensions. If you look at a molecular model of a structure such as CHBrClF, you will notice that the carbon atom and any two of the four substituents attached to it form a plane that is perpendicular to the plane formed by that carbon and the other two groups. If you imagine the carbon and two of the groups attached to it in the plane of a page, then the other two project above and below the page. Figure 2 illustrates a convention that chemists have developed to convey this 3-dimensional information in two dimensions.
In the formulas shown in the figure, the H, C, and Br lie in the plane of the page. In the model of (S)-bromochlorofluoromethane,the solid wedge implies that the Cl atom projects in front of that plane. The dashed wedge indicates the F projects behind it. The positions of the Cl and F atoms are interchanged in the structure representing (R)-bromochlorofluoromethane. If you interchange the positions of any two atoms attached to a chiral carbon, the new structure represents the enantiomer of the original.
Exercise 2 The name of the compound shown above is 1-chloro-1-fluoroethane. Is the configuration at the chiral carbon R or S? R S
