Rule 2

If rule 1 cannot provide a logical priority sequence, then the next atom joined to the atom adjacent to the chiral atom is used in the same manner. The sequence starts with the element with the highest atomic number (which in the example given in figure 15 is a chlorine atom), the second is the ethyl group (because the carbon has a carbon attached to it whereas the methyl carbon has only hydrogen atoms attached to it) and the remaining priorities follow the first rule (i.e. the methyl group followed by hydrogen).

Figure 15. Examples of Different Priority Sequences

In the second example in figure 15, chlorine has, again the highest atomic number and, thus, has the highest priority. This is followed by the isopropyl group that has three carbon atoms attached to the chiral carbon. The third priority is the ethyl group which has only two carbons attached to the chiral carbon whilst the lowest priority, as would be expected is the hydrogen atom.

Rule 3

This rule deals with the manner in which double bonds are prioritized. If a double or triple bond is present the specific atom(s) attached to the double or triple bond are considered to be doubled or tripled respectively. Examples of this procedure are given in figure 16.

Figure 16. Priority Sequence of Multiple Bonds

This concept, however, can require quite complicated analysis. In figure 17 the priority sequence of glyceraldehyde is considered. It is seen that the O, O, H of the equivalent aldehyde group takes priority over the O, H, H of the CH₂OH group. Thus, the priority sequence of the groups around the chiral carbon atom is, OH, CHO, CH₂OH, H.

Figure 17. The Priority Sequence of Glyceraldehyde.