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According to the first point of difference rule, second one should be correct right?

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marked as duplicate by Loong organic-chemistry Jun 10 at 15:33

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  • $\begingroup$ The name on the right is incorrect because it gives the locant 2 to the first substituent, whereas the one on the left gives locant 1 to the first substituent. $\endgroup$ – ralk912 Mar 20 '18 at 7:59
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    $\begingroup$ Please note that the ‘lowest sum rule’ that is mentioned in the picture does not exist. See also: chemistry.stackexchange.com/a/28009/7951 $\endgroup$ – Loong Mar 20 '18 at 12:34
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First of all, the ‘lowest sum rule’ mentioned in the picture in the question does not exist in IUPAC nomenclature.

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:

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;

(…)

The compound that is given in the question doesn’t have any principal characteristic group that could be expressed as a suffix. Therefore, Rule (c) is not relevant in this case.

Thus, a low locant is assgined first to the double bond according to Rule (e). Therefore, the name of the parent structure without any further substituents is cyclohex-1-ene. (Without further substituents, however, the locant ‘1’ would be omitted.)

Finally, low locants are assigned to the remaining substituents as a set according to Rule (f).

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; for example, the locant set ‘2,3,5,8’ is lower than ‘3,4,6,8’ and ‘2,4,5,7’.

Thus, the correct name is 1,6-dimethylcyclohex-1-ene rather than 2,3-dimethylcyclohex-1-ene since the locant set ‘1,6’ is lower than ‘2,3’.

1,6-dimethylcyclohex-1-ene

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  • $\begingroup$ @ Loong +1 for elaboration.In several of your posts ,you have quoted Blue Book .Would it be possible to obtain pdf for the same.If possible a link would help. $\endgroup$ – Chakravarthy Kalyan Jun 10 at 13:20
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The highest priority is double bond for this compound. And then consider to give lowest sum for substituents on double bond. Here is only one substituent present on double bond (methyl group), hence it should get the lowest number. Therefore, 1,6-dimethylcyclohex-1-ene is the correct name (1,6-dimethyl-1-cyclohexene is also correct). In either way, note that there should not have a space between 1,6-dimethyl- and cyclohex-1-ene.

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No, the right one is incorrect as the numbering starts at the double bond, it is suppose to start at the first substituent.

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