For the following compound:

enter image description here

I understand that the longest chain is of five carbon atoms. However, I am having trouble assigning the locants here.

Going bottom to top we get the locant set as: 223.
Going top to bottom we get the locant set as: 244.

Obviously, there are no functional groups in this compound so the first point of difference rule should govern the numbering of locants. Hence, according to me, bottom-to-top locant assignment is correct here.

Unfortunately, ChemSketch uses top-to-bottom assignment :(

What rule am I missing here?

I figure out the unsaturation is being given preference here, but I do not know any rule regarding it.

My IUPAC name: 2-bromo-2-methylpent-3-ene.
ChemSketch's name: 4-bromo-4-methylpent-2-ene

  • 2
    $\begingroup$ Double bond is prioritized over halogens. Besides, ChemSketch does generate IUPAC name by default, and "My IUPAC name" is a somewhat confusing term. It can be "My little pony", but "My IUPAC name" sounds weird to me. $\endgroup$
    – andselisk
    Jan 7, 2018 at 16:28
  • 1
    $\begingroup$ @andselisk "My IUPAC name is a somewhat confusing term" yeah obviously it is! I realized it now. Though given the context I hope it can be safely be assumed to mean that "The IUPAC name I expected it to be" without meaning that "My assigned name is Gaurang Tandon; My IUPAC name is 2-bromo-2-methylpent-3-ene" xD Anyway, regarding the first part, "Double bond is prioritized over halogens" I didn't know that. So unsaturation does not follow the first point of difference rule? $\endgroup$ Jan 7, 2018 at 16:34

1 Answer 1


It is correct that the principal characteristic group (the characteristic group chosen for citation at the end of a name by means of a suffix or a class name, or implied by a trivial name) has a higher priority for numbering than any other substituents (expressed as prefixes). It is also correct that the compound that is given in the question does not have any principal characteristic group.

However, you are missing the priority for unsaturation (i.e. multiple bonds), which is between the priority of the principal characteristic group. The most important simplified criteria for the numbering of simple acyclic compounds are:

  1. lower locants for suffixes
  2. lower locants for multiple bonds
  3. lower locants for prefixes
  4. lower locants for substituents cited first as a prefix in the name

The corresponding actual wording in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:


When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:


(c) principal characteristic groups and free valences (suffixes);


(e) saturation/unsaturation:

  (i) low locants are given to hydro/dehydro prefixes (…) and ‘ene’ and ‘yne’ endings;

  (ii) low locants are given first to multiple bonds as a set and then to double bonds (…);

(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;

(g) lowest locants for the substituent cited first as a prefix in the name;


Since the compound that is given in the question does not have any principal characteristic group, the first criterion (greater number of suffixes) given in P-14.4 (c) is not relevant in this case.

According to (e), the correct name is (2⁠E)-4-bromo-4-methylpent-2-ene and not (3⁠E)-2-bromo-2-methylpent-3-ene since the locant ‘2’ for the double bond is lower than ‘3’.


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