I'm trying to apply IUPAC rules to name the compound below.

TLDR: Why is this 3-ethyl-2,3,4-trimethylpentane and not 2,3-dimethyl-3-(1-methylethyl)-pentane?

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  1. Is this the correct source to use?
  2. According to above, p. 60:

P- The longest chain is numbered from one end to the other by arabic numbers, the direction being chosen so as to give the lower locants to the substituent groups (side chains) [see P-14.4(f)]. The lower set of locants is defined as the set that, when compared term by term with other locant sets cited in order of increasing magnitude, has the lower term at the first point of difference (see P-14.3.5).

The longest chain possible here is a pentane chain, and ChemSketch says that this is 3-ethyl-2,3,4-trimethylpentane.

  1. However, if I want to select the lowest set of locants, wouldn't 2,3-dimethyl-3-(1-methylethyl)-pentane be preferred, as its ordered set (2,3,3) is lower than (2,3,3,4)?

  2. Also, is 1-methylethyl preferable to prop-2-yl?

That doesn't affect 3., as it would be 2,3-dimethyl-3-(prop-2-yl)-pentane.

  1. Am I correct in assuming that prop-2-yl is preferable to isopropyl?

Otherwise, that would make it 2,3-dimethyl-3-isopropylpentane.

  • 1
    $\begingroup$ 1. Yes, 2. yes, 3. no, 4. no, 5. yes $\endgroup$
    – user7951
    Commented Oct 4, 2018 at 17:51

1 Answer 1


Answers to numbered questions:

1.  Yes (with the errata applied)
3.  Yes, however before the lowest locant set rule (P-45.2.2), maximum number of substituents as prefixes rule (P-45.2.1) is to be used. (See e.g. Alkane nomenclature - choosing the parent chain - maximize or minimize substituents?)
4–5. No, “prop-2-yl” is an incorrect spelling of propan-2-yl (P-, which is preferred to both isopropyl and 1-methylethyl (P-


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