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This is 1,6-dimethyl-cyclohexene. Why cannot this same structure have the IUPAC name 1,2-dimethyl-cyclohexene. In the given picture the locant sum comes to 7 but in my case it comes to 3, implying that my answer is in accordance with the rules. Aren't the alkyl groups supposed to be given the lowest number as per lowest locant sum rule?

Please do clarify. Thanks!

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  • $\begingroup$ Look at this maybe it can help :) $\endgroup$ – ParaH2 Mar 25 '18 at 16:00
  • $\begingroup$ How does the double bond influence the numbering? $\endgroup$ – J_B892 Mar 25 '18 at 16:05
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    $\begingroup$ Note that the widely-cited "sum of locants" rule is incorrect. See: IUPAC nomenclature: “Smallest sum of locants”? and the answers therein. $\endgroup$ – orthocresol Mar 25 '18 at 16:10
  • $\begingroup$ So like as in unsaturated compounds, in alicyclic compounds, the higher sum of locant structure is the correct one $\endgroup$ – J_B892 Mar 25 '18 at 16:11
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    $\begingroup$ Because the -ene must have the lowest sum of locants. We are naming a (cyclic) alkene, after all. Your suggestion would have otherwise the methyl groups at both sides of the double bond. $\endgroup$ – Alchimista Mar 25 '18 at 16:13
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Between alkenyl and alkyl groups, alkenyl groups (double bonds) have seniority over simple alkyl substituents groups such as methyl and ethyl groups (See the new version of Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 (Prepared by Henri A Favre & Warren H Powell); Royal Society of Chemistry, Cambridge, UK, December 2013, ISBN: 978-0-85404-182-4:

Briefly, it suggests "When several structural features appear in cyclic and acyclic compounds," low locants (numbering) are assigned to saturated/unsaturated groups in the following decreasing order of seniority: (a) low locants are given to ‘ene’ and ‘yne’ endings; (b) low locants are given first to multiple bonds as a set and then to double bonds; and (c) detachable alphabetized prefixes, all considered together in a series of increasing numerical order.

Thus, ene-group on cyclohexene get the highest priority such as cyclohex-1-ene (or, 1-cyclohexene). Since the double bond has the highest priority, the substituents on it (if any) should get the next priority such that it should have been 1-methylcyclohex-1-ene (or, 1-methyl-1-cyclohexene), if there are no other substituents. But in your molecule, there is an another methyl group present. Therefore that has the lowest priority. Since 1,2-numbers belongs to the double bond as mentioned above, that methyl can have been only 6 with numbering. Thus, the compound is: 1,6-dimethylcyclohex-1-ene (or, 1,6-dimethyl-1-cyclohexene).

Note that 1-methylcyclohex-1-ene has higher priority over 2-methylcyclohex-1-ene (lowest locant rule within double bond). Therefore, naming the molecule as 2,3-dimethylcyclohex-1-ene (or, 2,3-dimethyl-1-cyclohexene) is also not possible within IUPAC rules.

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  • $\begingroup$ If an alkyl group is there attached to a cycloalkenyl group then the double bond always gets the least possible locant numbering.Right? $\endgroup$ – J_B892 Mar 26 '18 at 4:50
  • $\begingroup$ Yes, but lowest number goes to the most substituted $\ce {sp^2}$ carbon (if double bond has the highest priority). $\endgroup$ – Mathew Mahindaratne Mar 26 '18 at 6:14

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