I am unable to share the image, so sorry; but can you tell me which is the correct name from these 2 of the same compound.

  • 3-(1-chloromethyl)-2-(1-methylethyl)-pentan-1-ol


  • 4-chloro-3-ethyl-2-(1-methylethyl)-butan-1-ol

Shouldn't the first one be correct as it's the longest chain with a functional group? According to IUPAC we don't care about substitution groups while selecting the longest chain at all, right?


1 Answer 1


You are right to consider the pentane chain of higher seniority than the butane. Indeed, the two names differ in the perception of the primary chain, apparent if the names are converted into the corresponding structures (the upper line in the illustration below):

enter image description here

With this in mind, you may assign the names of the two formulae about the same molecular structure.

Note: In case of the chloromethyl group, there is no other place of attachement for chlorine than on carbon-1 (for this side chain); hence, 1-chloromethyl can be substituted by chloromethyl. Second, 1-methylethyl is not wrong per sé, yet the motif frequently is faster to grasp (for the trained eye) as isopropyl (e.g., isopropyl alcohol [colloquial isopropanol]).

Side note: resources for learning chemistry includes references about nomenclature.

  • 1
    $\begingroup$ Note that the preferred IUPAC name for "3-(chloromethyl)-2-isopropylpentan-1-ol" is 3-(chloromethyl)-2-(propan-2-yl)pentan-1-ol. Also note that "isopropanol" is considered a bad name in nomenclature since there is no alkane "isopropane". $\endgroup$
    – Loong
    Jan 19, 2023 at 18:57
  • $\begingroup$ @Loong Moss' compilation, the full-pdf revised by 1 April 2022 states "P- The prefixes isopropyl, isopropylidene, and trityl are retained for use in general nomenclature but no substitution of any kind is allowed." with the example of the isopropylidene side chain. The term "isopropyl" equally is used by copy-paste of the structures into ChemDraw JS and twice by ChemDoodle .../... $\endgroup$
    – Buttonwood
    Jan 19, 2023 at 23:33
  • $\begingroup$ @Loong, 2 In case of ChemDoodle, on can pick the rule set to assign a name to a structure (guide, though "Attempted PIN" (preferred IUPAC name, pdf for version 11/12, section 14.2.1 Types of Names, p. 338). Though neither isopropanol, nor isopropyl alcohol are good names, they are still alive in the catalogues of Sigma, Fisher, Merck next to propan-2-ol. $\endgroup$
    – Buttonwood
    Jan 19, 2023 at 23:45
  • 2
    $\begingroup$ That's exactly what I said. "retained for use in general nomenclature" means "not the preferred IUPAC name" $\endgroup$
    – Loong
    Jan 20, 2023 at 17:04
  • 2
    $\begingroup$ @Martin-マーチン Since Rule P-44.1.1 (greater number of substituents corresponding to the principal characteristic group, which is expressed as suffix) comes before Rule P-44.3.2 (longest chain), a parent structure that includes the $\ce{-OH}$ group wins. In this case 3-(chloromethyl)-2-(propan-2-yl)pentan-1-ol. $\endgroup$
    – Loong
    Mar 10, 2023 at 21:05

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