Both names 4-formylpentanoic acid and 4-methyl-5-oxopentanoic acid are correct. In particular, the following rules of Section P-44 apply to both names:
- The parent structure has the maximum number of substituents corresponding to the principal characteristic group, which is expressed as a suffix (-oic acid), in accordance with Rule P-44.1.1.
- The principal chain has the greater number of skeletal atoms in accordance with Rule P-44.3.2.
- The principal chain has the lower locant for an attached group expressed as a suffix in accordance with Rule P-44.4.1 (h).
Therefore, we have to consider the Rules of Section P-45 in order to figure out which principal chain will lead to the preferred IUPAC name.
P-45 SELECTION OF PREFERRED IUPAC NAMES
Two or more names may result based on the same senior parent structure selected
according to P-44, because of different substitution patterns or multiple ccurrences of the same senior parent structure. A parent structure is defined (P -15.1) as a parent hydride, for example, benzene, a functionalized parent hydride, for example
cyclohexanol, or a functional parent compound, for example, acetic acid. Application of the criteria in this section will generate the preferred IUPAC name.
The first relevant rule of Section P-45 is about the number of substituents cited as prefixes:
P-45.2.1 The preferred IUPAC name is based on the senior parent structure that has the maximum number of substituents cited as prefixes (other than ‘hydro/dehydro’) to the parent structure.
Therefore, the preferred IUPAC name is 4-methyl-5-oxopentanoic acid (two substituents) rather than 4-formylpentanoic acid (one substituent).
The situation would be reversed if we place more substituents on the other chain, e.g. 5,5-dichloro-4-formylpentanoic acid (three substituents) rather than 4-(dichloromethyl)-5-oxopentanoic acid (two substituents).