KEY CONCEPT
When assigning priority, look only at the first atom attached to the chiral carbon, not at the
group as a whole. The higher the atomic number of this first atom, the higher the priority—
this same system is used to determine priority for both (E) and (Z) forms as well as (R) and (S)
forms.Step 2 (Classic Version): Arrange in Space
Orient the molecule in three-dimensional space so that the atom with the lowest priority (usually a
hydrogen atom) is at the back of the molecule. Another way to think of this is to arrange the point of
view so that the line of sight proceeds down the bond from the asymmetrical carbon atom (the
chiral center) to the substituent with lowest priority. The three substituents with higher priority
should then radiate out from the central carbon, coming out of the page, as shown in Figure 2.17.
Figure 2.17. Placing the Lowest-Priority Group in the BackStep 2 (Modified Version): Invert the Stereochemistry
If it is difficult to visualize rotating three-dimensional structures, one can simplify this process by
remembering one simple rule: any time two groups are switched on a chiral carbon, the
stereochemistry is inverted. By this logic, we can simply switch the lowest-priority group with the
group at the back of the molecule (the substituent projecting into the page). We can then proceed
to Step 3, keeping in mind that we have now changed the molecule to the opposite configuration.
Therefore, if we use this modified step, we need to remember to switch our final answer (either (R)
to (S), or (S) to (R)). This is a strategy we’ll commonly use on Fischer diagrams, as described below.