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ChemSketch generates the IUPAC name of the compound above as "2-bromo-1-chloro-5-fluoro-3-iodobenzene" but I have a different answer.

I know that I have asked a related question before as well. The key takeaway from orthocresol's answer was that:

first, order the prefixes by alphabetical order, and then choose the locant set in order to minimise the locants

I have applied the same here: the prefixes ordered in alphabetical order are "bromo, chloro, fluoro, iodo". Now, I have to choose a locant set that minimizes the locants. A possible locant set is "1-bromo-2-chloro-4-fluoro-6-iodo" which correctly locates the substituents. Also, the locant set "1246" is lower than "2153", as noted in this answer. So, according to me, the preferred IUPAC name should be "1-bromo-2-chloro-4-fluoro-6-iodobenzene".

What exactly am I doing wrong here?


1 Answer 1


The locant set ‘1,2,4,6’ is not lower than ‘2,1,5,3’.

According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the lowest locant set is defined as follows:

P-14.3.5 Lowest set of locants

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’.

Especially note that the locants are cited in order of increasing value when they are compared.

Thus, the locant set ‘2,1,5,3’ is regarded as ‘1,2,3,5’, which is lower than ‘1,2,4,6’.

Therefore, the correct name is 2-bromo-1-chloro-5-fluoro-3-iodobenzene.


  • $\begingroup$ Thanks that'll do! I understand OC is supposed to be systematic but I don't understand why they couldn't make rules easier to follow in a simple language. $\endgroup$ Feb 14, 2018 at 7:31
  • $\begingroup$ @GaurangTandon Hm, I checked the previous definition in the 1998 IUPAC Guide Principles of Chemical Nomenclature, but it isn’t much different: “(…) Lowest locants are determined by comparing alternative sets of locants. When compared term-by-term with other locant sets, each in order of increasing magnitude, a set of lowest locants has the lowest term at the first point of difference; (…)” $\endgroup$
    – user7951
    Feb 14, 2018 at 7:42
  • $\begingroup$ I didn't mean to refer to the previous edition of nomenclature. I meant to say that these nomenclature guides are written in a complicated manner that presents a steep learning curve for beginners to master it. $\endgroup$ Feb 14, 2018 at 7:52
  • 5
    $\begingroup$ @Gurang Tandon it as such because uniquely name every single molecule in a virtually infinite set of molecules is a big task :) $\endgroup$
    – Alchimista
    Feb 14, 2018 at 12:07

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