Ethyl 2-methyl-5-oxocyclopentane-1-carboxylate


The rules say that functional groups are given more priority than side chains but if there is more than one functional groups then we consult the priority table for nomenclature...

Hence the ester group is given more priority over the ketone group. But then while naming the cyclopentyl group, shouldn't we give the ketone group the smaller number than the methyl group? Why didn't we call it ...5-methyl-2-oxocyclopentane...?

  • $\begingroup$ I think your argument is correct. Pubchem names may not always be correct. For example, look here for correct related name. $\endgroup$ Commented Nov 30, 2022 at 18:47
  • $\begingroup$ @Loong please take a look at this name: achemblock.com/… $\endgroup$ Commented Dec 1, 2022 at 4:36
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    $\begingroup$ @AgrimTripathi The name ethyl 4,4-dimethyl-2-oxocyclopentane-1-carboxylate is correct; however, it's not related to your question. $\endgroup$
    – Loong
    Commented Dec 1, 2022 at 18:00
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    $\begingroup$ @Loong how is it not related? It literally contains the exact same functional groups and prioritizes them in the way one would expect. Ester > Ketone > Methyl Instead of the Ester > Methyl > Ketone prioritization in the original post. I think Mathew is correct in saying it's a Pubchem naming error. $\endgroup$ Commented Dec 1, 2022 at 20:34
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    $\begingroup$ @GrundleButt There is no order of functional group priority for substituents that are cited as prefixes in the name. The name ethyl 4,4-dimethyl-2-oxocyclopentane-1-carboxylate is not related to the question since it can be easily explained in the usual way using Rule P-14.4 (f). This rule, however, doesn't explain why ethyl 2-methyl-5-oxocyclopentane-1-carboxylate is correct; you need to consider Rule P-14.4 (g) in this case. $\endgroup$
    – Loong
    Commented Dec 2, 2022 at 6:41

1 Answer 1


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:

P-14.4 NUMBERING 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;


For the compound that is given in the question, you have correctly identified the principal characteristic group as ester. It is expressed as suffix ‘carboxylate’. Thus, a low locant is assigned first to the ester according to Rule (c), which yields ‘…1-carboxylate’.

The structure doesn’t have multiple bonds that wound be expressed using ‘ene’ or ‘yne’ endings. Thus, Rule (e) is not relevant in this case.

Next, low locants are given to any substituents that are expressed as prefixes (e.g. ‘chloro’, ‘methyl’, or ‘oxo’) according to Rule (f). Note that all prefixes are considered together; i.e., there isn’t any order of functional group priorities at this point. In this case, however, two names are possible, ‘ethyl 2-methyl-5-oxocyclopentane-1-carboxylate’ as well as ‘ethyl 5-methyl-2-oxocyclopentane-1-carboxylate’, since the locant set ‘2,5’ for all prefixes considered together in a series of increasing numerical order is the same for both names. Therefore, Rule (f) is not sufficient to decide which name is correct in this case.

Finally, the lowest locant is given to the substituent cited first as a prefix in the name according to Rule (g); i.e., the locant ‘2’ is given to the methyl group since it is cited first.

Therefore, the complete name is ethyl 2-methyl-5-oxocyclopentane-1-carboxylate.

ethyl 2-methyl-5-oxocyclopentane-1-carboxylate

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