While answering a question about keto-enol tautomerism the question arose, how can I refer to the resonance hybrid instead of on of the resonance forms. In the case of the deprotonated butane-2-one, I can name 1 3-oxobutan-2-ide and 2 but-2-en-olate (ignoring stereochemistry). I cannot come up with a name for the hybrid 3.

resonance structures and names

While this might be quite a simple case, there are many resonance stabilised structures out there, which have contributions from more than one form. It gets especially complicated for ionic substructures. I wondered if there is either already an official recommendation (preferred IUPAC name), or any other suggestion at all handling those cases. After all, we are treating the same bonding situation with two different names, there should be something that covers both (multiple) cases.


Nomenclature of Organic Chemistry, IUPAC Recommendations and Preferred names 2013 in the section P-76 DELOCALIZED RADICALS AND IONS mentions only "totally delocalized" ions like cyclopentadiene derived cyclopentadienyl radical, cyclopentadienylium cation, cyclopentadienide anion; benzo[7]annulenylium (azulenium?) and pentadienyl radical.

However, there is a draft having this chapter expanded, allowing more systematic naming of delocalized ions.

P-76.1.2 Partial delocalization is denoted by the descriptor deloc preceded by the locants indicating the extent of delocalization. This descriptor and its locants are cited immediately before the appropriate suffix and enclosed in parentheses.
delocalized anion example structure image

But I am not 100% sure how exactly would that apply to your 3 structure.. let's be brave and creative:

(2Z)-but-2-en-2-(2,3,O-deloc)olate or
(2Z)-but-2-en-2-ol(2,3,O-deloc)ate or if you don't need stereochemistry
3-oxobutan-2-(2,3,O-deloc)ide or maybe even just
butanon(2,3,O-deloc)ide ? .. or
butanon(O,2,3,-deloc)ide ?

However, that draft is from 2004, but this rule did not make it into the current official nomenclature (2013), so for now we can just speculate, or contribute to the upcoming version.

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  • $\begingroup$ Thank you very much for digging this up. This is indeed an intriguing proposal. I guess one of the principal shortcomings in this rule is which ending takes precedence when a hetero-atom is involved. $\endgroup$ – Martin - マーチン Apr 27 '17 at 4:49

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