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Why is the following compound named as 4-bromo-2-methoxyphenol and not 4-bromo-6-methoxyphenol? Shouldn't the substituents be numbered according to their alphabetical order?

Structure of 4-bromo-2-methoxyphenol

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When choosing which name to use when the only difference is the numbering, we choose the combination that gives the lowest possible numbers, 2 and 4 in this case.
So the name will be 4-bromo-2-methoxyphenol. The substituents are given in alphabetical order.

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  • $\begingroup$ A more explicit last line would be "The substituents are written down in alphabetical order". $\endgroup$ – William R. Ebenezer Aug 22 at 11:58
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The most important simplified criteria for the numbering in such cases are:

  1. lower locants for the principal characteristic groups, which are expressed as a suffix or implied by the name of the functional parent compound
  2. lower locants for prefixes
  3. 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);

(…)

(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 given in the question has the functional parent compound phenol. The name ‘phenol’ implies the presence of the characteristic group $\ce{-OH}$. Since there are no other characteristic groups and low locants are assigned first to the principal characteristic group, the locant ‘1’ is assigned to the substituent group $\ce{-OH}$.

Finally, low locants are assigned to the remaining substituents as a set according to Rule (f); i.e. low locants are assigned to the prefixes ‘bromo’ and ‘methoxy’, all considered together as a set in a series of increasing numerical order. Thus, the systematic name for this compound is 4-bromo-2-methoxyphenol and not ‘4-bromo-6-methoxyphenol’ since the locant set ‘2,4’ is lower than ‘4,6’.

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’.

Only if no decision can be attained by application of Rule P-14.4(f), the lowest set of locants is selected according to P-14.4(g) in the order of the citation in the name (which indeed is based on the alphabetical order).

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