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As far as I know, both following names seem to be correct for the compound below. Please explain which IUPAC name is correct by which IUPAC rules:

4-formylpentanoic acid vs. 4-methyl-5-oxopentanoic acid

  • 4-formylpentanoic acid (methyl group counted in the main chain);
  • 4-methyl-5-oxopentanoic acid (aldehyde group counted in the main chain).
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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

P-45.0 INTRODUCTION

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).

4-methyl-5-oxopentanoic acid

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).

5,5-dichloro-4-formylpentanoic acid
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