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As far as I know, 'ethoxy' should be given the locant '1' as it is a functional group. The compound should thus be named 1-ethoxy-2,2-dimethylcyclohexane.

However, I was told that the correct name is 2-ethoxy-1,1-dimethylcyclohexane. Can anyone please explain why this is so? My teacher explained this to me in terms of the lowest locant sum rule, which I just discovered doesn't even exist.

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The smallest sum of locants rule does not exist in the IUPAC recommendations. The application of this ‘rule’ can lead to wrong results in many instances.

The corresponding section in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) actually reads as follows:

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’.
(…)

With regard to numbering of locants, simple prefixes (simple substituent groups consisting of just one part that describes an atom, or group of atoms as a unit, for example methyl and ethoxy) are considered together with equal seniority:

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:

(…)

(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;

(…)

Therefore, the example is named as 2-ethoxy-1,1-dimethylcyclohexane rather than 1-ethoxy-2,2-dimethylcyclohexane since the locant set ‘1,1,2’ is lower than ‘1,2,2’.

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  • $\begingroup$ "...simple substituent groups consisting of just one part that describes an atom, or group of atoms as a unit, for example methyl and ethoxy". Could you please give more such examples. Also, "... low locants are assigned to them in the following decreasing order of seniority: ...(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;" Could you mention the order as well? $\endgroup$ – agdhruv Jun 21 '15 at 11:32
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    $\begingroup$ @ag_dhruv In this context, ‘simple substituent groups’ are substituents that are cited as prefixes before the name of the parent structure and do not modify the structure (e.g. ‘bromo-’, ‘chloro-’, ‘methyl-’, ‘ethyl-’, and ‘nitro-’). $\endgroup$ – Loong Jun 21 '15 at 18:47
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    $\begingroup$ @ag_dhruv The ‘series of increasing numerical order’ mentioned in P-14.4 (f) corresponds to the order that is applied in P-14.3.5 above; e.g. ‘5-bromo-8-hydroxy-4-methylazulene-2-carboxylic acid’ has the locant set ‘2,4,5,8’ in a series of increasing numerical order. $\endgroup$ – Loong Jun 21 '15 at 18:47
  • $\begingroup$ Slightly irritating technicality, but according to P-14.4(f), the locant set under consideration is that of the "detachable alphabetized prefixes", hence in the example '5-bromo-8-hydroxy-4-methylazulene-2-carboxylic acid' it is '4,5,8' and not '2,4,5,8' (this is corrected in the errata). $\endgroup$ – orthocresol Aug 14 '18 at 15:15

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