1,6-dimethylcyclohexene and 2,3-dimethylcyclohexene

Question

Is there a difference between these two molecules?

If so how does one tell? What is the correct nomenclature for cycloalkenes? I know that the double bond is assumed to be between C1 and C2 but why can't we number a cyclohexene to get 2,3? Is it because a lower sum is not preferred but simply lower individual numbers if possible?

Answer 1

With regard to numbering of locants within the same compound class, multiple bonds have seniority over simple substituent groups (e.g. methyl, ethyl, etc.) that are used for naming of branched alkanes. On this matter, 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:

(…)

(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;

(…)

Note that Rule e takes precedence over Rule f.

For example, a preferred IUPAC name is 3-bromocyclohex-1-ene (not 1-bromocyclohex-2-ene): (This example is given in the Blue Book.)

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; (…)

Therefore, the structure given in the question is named as 1,6-dimethylcyclohex-1-ene rather than 2,3-dimethylcyclohex-1-ene since the locant set ‘1,6’ is lower than ‘2,3’.

Answer 2

The IUPAC rule is to have the first branch to have the lowest number, even if it gives you 1,6-dimethylcyclohexene.