In the first example below, if you add up the number of carbon which has a substituent, you will get 2 + 4 + 4 = 10. However, if you start from left to right, then you'd get 2 + 2 + 4 = 8. Why don't we follow the latter one when it gives us a lower number? (Lowest set of locant rule.)

The same applies for the second problem: we could've gotten 2 + 3 + 4, but we chose 2 + 4 + 5, why so?

1: 4‐chloro‐2,2‐dimethylpentane; 2: 4‐bromo‐5‐methylhex‐2‐ene

  • $\begingroup$ The substituents for haloalkanes are named alphabetically. Chloro- before methyl-. Then use the lowest number first? Searching IUPAC nomenclature for haloalkanes will return lots of web results $\endgroup$
    – LiamH
    Commented Aug 6, 2015 at 14:30
  • 6
    $\begingroup$ The lowest sum of locants rule does not exist in the IUPAC recommendations. $\endgroup$
    – user7951
    Commented Aug 6, 2015 at 18:23

1 Answer 1


Indeed, the first compound in your sketch is numbered incorrectly according to IUPAC. IUPAC rules for branched hydrocarbons say: "When series of locants containing the same number of terms are compared term by term, that series is "lowest" which contains the lowest number on the occasion of the first difference. This principle is applied irrespective of the nature of the substituents. " (emphasis mine). In your top case, this means that you have two numbering choices: either 2,2,4 or 2,4,4. The 2,2,4 choice has a lower locant at the first point of difference, so it is preferred. When you construct the name, you indicate the substituents in alphabetical order, so the full name of the top compound is 4-chloro-2,2-dimethylpentane.

If unsaturations are present, then these have a higher priority than alkyl and halogen substituents. From the rules for unsaturated hydrocarbons: "The chain is so numbered as to give the lowest possible numbers to the double bonds." This rule is sufficient to impose the numbering you report.

As Loong correctly pointed out, the links provided above refer to the text of the now obsolete 1979 recommendations. Both the 1979 and 1993 recommendations are available on the same site, maintained by ACD Labs with IUPAC's permission, and are in accordance with what shown above.

Unfortunately, the text of the most recent (2013) recommendations is not freely accessible (that I know of). This is a link to the electronic version of the book, published by The Royal Society of Chemistry. The relevant section is P-14.3.5 in Chapter P-1 (General Principles, Rules, and Conventions): "The lowest set of locants is defined as the set that, when compared term by term with other locant sets, each cited in order of increasing value, has the lowest term at the first point of difference; for example, the locant set '2,3,5,8' is lower than '3,4,6,8' and '2,4,5,7'."

  • $\begingroup$ But the numbering given above is correct (or at least it's given in the book). That is why I am confused. It should be 2,2,4, but in the book, the name 2-chloro-4,4-dimethylpentane is given correct, for some reason. $\endgroup$ Commented Aug 6, 2015 at 16:14
  • $\begingroup$ I'm afraid that the book you are referring to might contain a mistake then. Here is the CAS number for that structure: 33429-72-0. You can search the web for the name it refers to, in order to check that it is, in fact, 4-chloro-2,2-dimethylpentane, e.g. from NIH TOXNET, and from a CRC reference book. $\endgroup$
    – MarcoB
    Commented Aug 6, 2015 at 17:04
  • $\begingroup$ @MarcoB Please note that your links acdlabs.com/iupac/nomenclature/79/r79_36.htm and acdlabs.com/iupac/nomenclature/79/r79_53.htm lead to obsolete IUPAC recommendations of 1979. $\endgroup$
    – user7951
    Commented Aug 6, 2015 at 18:16
  • $\begingroup$ @Loong Yes, and the same site contains the text of the 1993 recommendations, which are in agreement on this point. I'll add a link to the newest 2013 recommendations, but unfortunately those are behind a paywall. Fortunately, however, the interpretation of the point under contention here has not changed throughout the years, although the 1979 recommendations were the most direct about it. $\endgroup$
    – MarcoB
    Commented Aug 6, 2015 at 18:35

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