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I strongly feel that the answer should be option (1) as the ester functional group has a greater preference over the double bond and hence it should get a lower locant. But the source of the question says that the answer is option (2). My question is why the double bond gets a lower locant than ester in this case? As far as I know the lowest set out locants rule is applicable for substituents but when the comparison is between functional groups the preference order plays a greater role. Please let me know if I am wrong.

  • $\begingroup$ related: Doubts about IUPAC nomenclature of 4-ethyl-1-fluoro-2-nitrobenzene // Very related: IUPAC nomenclature: "Smallest sum of locants"? // the lower set of locants is used, i.e. 2 is more correct than the others (but not the preferred IUPAC name either). $\endgroup$ Apr 24 '19 at 11:29
  • $\begingroup$ @Martin Lower set of locants is preferred when we are comparing only substituents but in my question preference order is between functional groups. $\endgroup$ Apr 24 '19 at 11:34
  • $\begingroup$ I considered 4-ethanoyloxycyclopen-2-ene as a prefix, I might be wrong; let's wait until the experts find this question. // Btw. You should edit this thought process into the question itself. $\endgroup$ Apr 24 '19 at 11:39
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    $\begingroup$ Related: chemistry.stackexchange.com/q/99458/7951 $\endgroup$
    – user7951
    Apr 24 '19 at 12:44
  • $\begingroup$ @Loong So that means leaving the principal functional group and unsaturations, rest all other groups which are used as prefixes are treated as substituents even if that is a functional group(not principal)? And in that case the unsaturation gets the immediate next preference after the principal functional group? $\endgroup$ Apr 24 '19 at 13:11

As already explained in a related answer, the most important simplified criteria for the numbering in such cases are:

  1. lower locants for the principal characteristic group that is expressed as suffix
  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;


The principal characteristic group is the carboxylic acid. Therefore, the lowest locant is assigned to the $\ce{-COOH}$ group in accordance with Rule (c).

Next, a low locant is assgined to the double bond according to Rule (e). Therefore, the name of the structure without any further substituents is cyclopent-2-ene-1-carboxylic acid.

Finally, a low locant is assgined to the remaining substituent to Rule (f).

Thus, the correct name is 4-(acetyloxy)cyclopent-2-ene-1-carboxylic acid.

4-(acetyloxy)cyclopent-2-ene-1-carboxylic acid


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