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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?
The most important simplified criteria for the numbering in such cases are:
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;
(…)
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.
