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Following compound is named as

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(1) 6-Mercaptocyclohex-4-ene-1,3-diol
(2) 1-Mercaptocyclohex-2-ene-4,6-diol
(3) 1-Mercaptocyclohex-5-ene-2,4-diol
(4) 4-Mercaptocyclohex-2-ene-1,5-diol

As per my knowledge if I start numbering the compound from the alcohol then to mercapto group, then alkene group.

So, according to me, the numbering should be from bottom alcohol as number 1 and making the way to the top alcohol as number 3. Then mercapto will come at number 4 and alkene at 5.

So as per me the name should be 4-mercaptocyclohex-5-ene-1,3-diol, but according to the answer key the answer should be (1). Is there any rule which I am missing out on?

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The most important simplified criteria for the numbering in such cases are:

  1. lower locants for the principal characteristic group that is expressed as suffix
  2. lower locants for multiple bonds
  3. lower locants for prefixes
  4. 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);

(…)

(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;

(g) lowest locants for the substituent cited first as a prefix in the name;

(…)

For the compound that is given in the question, the principal characteristic group is $\ce{-OH}$. It is expressed as suffix “ol”. Thus, low locants are assigned first to the $\ce{-OH}$ groups according to Rule (c), which yields “…1,3-diol”.

Next, a low locant is given to the double bond according to Rule (e), which gives “4-ene-1,3-diol”. This step also establishes the numbering of the other atoms of the ring so that there is no longer a choice for the last substituent. Nevertheless, strictly following the rules, a low locant can be assigned to the remaining substituent according to Rule (f).

Thus, the complete name is 6-sulfanylcyclohex-4-ene-1,3-diol. Note that the prefix “mercapto” is no longer recommended.

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