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Compound to be named

Is the IUPAC name of above compound 7,7-dibromo oct-5-yn-4-ol or 2,2 dibromo oct-3-yn-5-ol ?

I understand that the former name is given keeping in mind -OH as the functional group but by giving the latter name we will have the least sum of locants (ie, following lowest sum rule) .

When should the lowest sum rule be dropped ? Thank You in advance.

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    $\begingroup$ Daily dose of pedantry from me: Alkynes shouldn't be depicted as being bent like that. $\endgroup$ – orthocresol Apr 26 '16 at 16:35
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    $\begingroup$ 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. $\endgroup$ – Loong Apr 26 '16 at 16:39
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In this case, the relevant criteria for the numbering are:

  1. lower locants for suffixes (here: ‘ol’)
  2. lower locants for multiple bonds (here: ‘yne’)
  3. lower locants for prefixes (here: ‘bromo’)

The corresponding subsection 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;

(…)

Therefore, the correct name for the compound given in the question is ‘7,7-dibromooct-5-yn-4-ol’ (not ‘2,2-dibromooct-3-yn-5-ol’) since the locant ‘4’ for the suffix ‘ol’ is lower than ‘5’.

7,7-dibromooct-5-yn-4-ol

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