Following compound is named as
(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?
Answer
The most important simplified criteria for the numbering in such cases are:
- lower locants for the principal characteristic group that is expressed as suffix
- lower locants for multiple bonds
- lower locants for prefixes
- 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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