My book names this compound as 6-ethyl-5-methylcyclohex-2-en-1-one.

I have learnt that after numbering the principal functional group which is ketone in this case, we have to treat all other groups as substituents and therefore I think the name should be 2-ethyl-3-methylcyclohex-5-en-1-one.

Why should the double bond be given priority?

  • $\begingroup$ Not an answer, but can confirm that several sources independently suggest that 6-ethyl-5-methylcyclohex-2-en-1-one is the correct name. Note also that the italics/formatting used in your post is superfluous! $\endgroup$
    – NotEvans.
    Commented Jul 14, 2018 at 20:16
  • $\begingroup$ I do not see any reason why I should give priority to the double bond here. $\endgroup$ Commented Jul 14, 2018 at 20:18

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:


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;


You have correctly identified the principal characteristic group and assigned the lowest locant to the ketone 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 further substituents is cyclohex-2-en-1-one rather than cyclohex-5-en-1-one since the locant 2 for the double bond is lower than 5.

Thus, the correct name is 6-ethyl-5-methylcyclohex-2-en-1-one.


  • $\begingroup$ This helps. So While naming I must: Assign the lowest locant to the principal functional group and then look for double/multiple bonds and give them the next priority. Right? $\endgroup$ Commented Jul 14, 2018 at 20:38
  • 2
    $\begingroup$ @ParthChauhan Yes, for most simple organic compounds (in particular, compounds without added indicated hydrogen) this is correct. $\endgroup$
    – user7951
    Commented Jul 15, 2018 at 9:24

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