2
$\begingroup$

I'm afraid I can't find an image for this organic compound, but when applying lowest locant rule in the numbering (1-methylethenyl) gets 2 and Cl gets 4. (2,4) is less than (3,5). Why then is lowest locant rule not applied here? Also here (1-methylethenyl) is an alkene, and isn't priority order of alkenes before halogens which have no priority?

$\endgroup$
4
$\begingroup$

The most important simplified criteria for the numbering are:

  1. lower locants for the 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;

(…)

The principal characteristic group of the compound that is given in the question is the $\ce{-OH}$ group. It is expressed as the suffix “-ol”. According to Rule (c), a low locant is assigned first to this group, i.e. “-1-ol”.

In the next step, low locants are assigned to the double bonds according to Rule (e), i.e. “cyclopenta-1,3-dien-1-ol”.

That leaves only one possible locant set for the remaining substituents, i.e. “3-chloro” and “5-(prop-1-en-2-yl)”. The rules (f) and (g) are not needed to make a decision.

Therefore, the complete name for the compound that is given in the question is 3-chloro-5-(prop-1-en-2-yl)cyclopenta-1,3-dien-1-ol.

3-chloro-5-(prop-1-en-2-yl)cyclopenta-1,3-dien-1-ol

Note that the preferred prefix for the “isopropenyl” or “1-methylethenyl” (completely: “1-methyleth-1-en-1-yl”) is now “prop-1-en-2-yl”.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.