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I came across this question while practicing iupac nomenclature.

Should we number the parent chain from left to right (shown in blue) because there are three cholrine atoms attached to C1 while there are only two alcohol groups attached to C2?

Or should we start the numbering from the right side instead (shown in orange) as alcohol group is preferred over the chlorine group?

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  • $\begingroup$ Alcohol function has a preference over chloro function. Therefore numbering in orenge is the correct one (name ending with -1,1-diol). $\endgroup$ – Mathew Mahindaratne Apr 20 at 0:45
  • $\begingroup$ But isn't there a rule that says that numbering starts from the side that has more substituents? $\endgroup$ – Chahat Kalsi Apr 20 at 5:45
  • $\begingroup$ Look here for related answer. $\endgroup$ – Mathew Mahindaratne Apr 20 at 16:26
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Concerning the numbering of locants for substituents, the most important simplified criteria are:

  1. lower locants for the principal characteristic group that is expressed as suffix
  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 $\ce{-OH}$ group is the principal characteristic group in this case. It is expressed as suffix ‘ol’.

Therefore, low locants are assigned first to the $\ce{-OH}$ groups according to Rule (c), which yields ethane-1,1-diol.

After that, a low locant is assigned to the chloro groups according to Rule (f); however, the only possible numbering left is 2,2,2-trichloroethane-1,1-diol.

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