Figure 1.2. Numbering the Longest Carbon Chain
The highest-priority functional group should have the lowest possible number; if all substituents
have the same priority, make their numbers as low as possible.After we have discussed the functional groups most commonly tested on the MCAT, we’ll review a
table of those functional groups in order of priority. For now, keep in mind that the more oxidized
the carbon is, the higher priority it has in the molecule. Oxidation state increases with more bonds
to heteroatoms (atoms besides carbon and hydrogen, like oxygen, nitrogen, phosphorus, or
halogens) and decreases with more bonds to hydrogen.
Just like straight chains, rings are numbered starting at the point of greatest substitution,
continuing in the direction that gives the lowest numbers to the highest-priority functional groups.
Somewhat counterintuitively, if there is a tie between assigning priority in a molecule with double
and triple bonds, the double bond takes precedence.
3. Name the Substituents
Substituents are functional groups that are not part of the parent chain. A substituent’s name will
be placed at the beginning of the compound name as a prefix, followed by the name of the longest
chain. Remember that only the highest-priority functional group will determine the suffix for the
compound and must be part of the parent chain.
Carbon chain substituents are named like alkanes, with the suffix –yl replacing –ane. The prefix n–
that we see in Figure 1.3 on n-propyl simply indicates that this is “normal”—in other words, a
straight-chain alkane. Because this prefix will not always be present, it is safe to assume that alkane
substituents will be normal unless otherwise specified.
Figure 1.3. Common Normal Alkyl Substituents
The bond on the right side of each substituent connects to the parent molecule.