Today I was considering to answer the question "Which tautomer of hexane-2,4-dione is more stable?" I wanted to run a few calculations and while preparing a directory structure I wanted to make sure that I used correct and unique names for the compounds. I am citing the picture in the named question:

scheme of tautomers

I was going to derive the names of the structures from the parent compound hexane-2,4-dione. Hence I would have named compound 1 (Z)-2-hydroxyhex-2-en-4-one and 2 (Z)-4-hydroxyhex-3en-2-one.

When I recreated the above picture in ChemBioDraw and used it to autogenerate the name, the result for 1 was different: (Z)-5-hydroxyhex-4-en-3-one. See below.

scheme of tautomers with generated names

I am no expert on nomenclature and I do not have the blue book handy. Usually I rely on the names generated through software. Most of the times they are accurate enough to name a compound unambiguously and that is often what I am going for.
I understood from this answer, P-14.3.5 Lowest set of locants, that I should use the 4,2,2 set over the 5,3,2.

Is my thinking correct and the auto-generated names are wrong, or is the software correct and my thinking is wrong?


You numbered all locants together in a series and you found the lowest set of locants according to Subsection P-14.3.5 of the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) since the locant set ‘2,2,4’ is lower than ‘3,4,5’.

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; (…)

This approach would be correct if all locants refer to detachable prefixes (like ‘2-hydroxy’). However, in the given compounds, the locants refer to prefixes (‘hydroxy’), endings (‘ene’), and suffixes (‘one’). The assignment of low locants to such different features is stipulated in Subsection P-14.4 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;


Therefore, the resulting name is (4​Z)-5-hydroxyhex-4-en-3-one (not (2​Z)-2-hydroxyhex-2-en-4-one) since the locant ‘3-one’ for the principal characteristic groups is lower than ‘4-one’.



You will assign lowest locant to the group with highest priority. Here priority order is Keto>hydroxy>alkene. So use that numbering which gives lowest locant to keto group. So I think the names given by software are correct.

  • 1
    $\begingroup$ I would really appreciate if you could back that up with the official sources, hence the reference request. $\endgroup$ Jan 27 '16 at 13:12
  • $\begingroup$ Ok. Sure. I will definitely try to provide a good reference. That's the logic I have learnt and it also matches with software. $\endgroup$ Jan 27 '16 at 13:14
  • $\begingroup$ One of the reference is 'Organic Chemistry' by John E. Mcmurry. There is an article in the book stepwise describing nomenclature of polyfunctional compounds. It also gives a priority order for common functional groups $\endgroup$ Jan 27 '16 at 13:19

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