# Formal nomenclature for fulvene

I am a little bit confused trying to name the compound fulvene:

Fulvene consists of a cyclopentadiene ring and a $$\ce{{}=CH2}$$ substituent group. According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the prefix ‘methylene’ is no longer recommended to designate the $$\ce{{}=CH2}$$ group. The preferred prefix is ‘methylidene’; thus, the systematic name of fulvene without locants is methylidenecyclopentadiene.

Concerning the numbering of locants, the current IUPAC recommendations read 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:

(…)

(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;

(…)

Therefore, low locants are given first to the double bonds in the ring according to (e) and then to the prefix ‘methylidene’ according to (f). Thus, the correct systematic name is 5-methylidenecyclopenta-1,3-diene rather than 3-methylidenecyclopenta-1,4-diene since the locant set ‘1,3’ for the double bonds is lower than ‘1,4’.

Note that according to IUPAC nomenclature since 1993, locants are placed immediately before that part of the name to which they relate (e.g. cyclopenta-1,3-diene rather than 1,3-cyclopentadiene).

• @hBy2Py - Retracted close vote. Note: This is something that gets sorted out in a Google search in one click of the button - the first hit is Wikipedia, the third is PubChem, the fourth is ChemSpider (the second is the Wikipedia entry for Fulvenes). If we're going to be a proxy for an internet search engine, we probably ought to discuss it in Meta. – Todd Minehardt Feb 18 '17 at 4:06
• @ToddMinehardt You make a good point, but I feel all of the various nomenclature answers here add something that often is missing in resources like those resources you turned up in Google: explanation. Answerers here (Loong, often) typically go to considerable effort to explain why a given name fits the compound of the question. Much more valuable to the OPs and to future readers. – hBy2Py Feb 18 '17 at 4:39