# What is the IUPAC nomenclature for the cyclic mono-alkene ions formed after proton or hydride abstraction?

In this blog post, Steven Bachrach discusses the (anti-)aromaticity or lack thereof in the homologous series of anionic and cationic cyclic mono-alkenes formed respectively by deprotonation and hydride abstraction at a vicinal carbon:

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Bachrach uses the construction -enyl anion/cation to name these species, which seems sensible to me. However, I also vaguely recall seeing generalized use of the -ide and -ium suffixes for singly charged anions and cations. For example, I know that $\ce{NO+}$ is nitrosonium, but I seem to be recalling incorrectly that $\ce{NO-}$ is nitroxide. $($The NIST Webbook page only calls $\ce{NO-}$ the nitric oxide anion, and the Wikipedia article on nitroxide-mediated radical polymerization is discussing radical alkyl N-oxides $(\ce{R2NO^\bullet})$, not $\ce{NO-}$.$)$ On the other hand, $\ce{R3CO-}$ ions are unequivocally alkoxides.

So: What are the systematic IUPAC names for these compounds?

• If you search 'cyclohepatrienyl cation' you will find that it's synonymous with tropilium. I tried searching for 'cyclopropenyl cation' and got no synonyms... Maybe worth trying them one by one, and you may stumble in some document describing the whole series. – user6376297 Jul 8 '17 at 20:03
• @user6376297 Huh, good thinking. Tropylium sounds suspiciously like a historically-derived name, though, so I'm guessing any systematic naming won't be related to it. Not sure, though. – hBy2Py Jul 8 '17 at 20:07
• Why does the first row of examples follow the pattern of 1a, 2a, 3a ...; yet the second 1c, 2c, 3c, ... rather than 1b, 2b, 3b, ... ? – Buttonwood Jul 8 '17 at 21:41
• @Buttonwood #a is for anions, #c is for cations. – hBy2Py Jul 8 '17 at 21:49
• Also Is there a preferred way of naming the resonance hybrid in keto-enol tautomerism? might not be unrelated. – mykhal Jul 9 '17 at 9:17

## Anions

In general nomenclature, anions may be named by using functional class nomenclature; however, the resulting names are not the preferred IUPAC names. The corresponding subsection of the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:

P-72.2.1 Functional class nomenclature

Functional class nomenclature can be used, in general nomenclature, to describe anionic compounds. An anion that can be considered as derived formally by adding an electron to a radical may also be named by adding the class name ‘anion’ as a separate word to the name of the substituent group. The names are formed by using the names of corresponding radicals (not necessarily the name of substituent groups) and the class name ‘anion’ as a separate word. The multiplying prefixes ‘di’, ‘tri’, etc., are added to the class name to denote multiple anions. This type of nomenclature is limited to anions having anionic centers in the same structure. Systematic names (see P-72.2.2) are preferred IUPAC names.

The functional class names for the anions that are given in the question are:

• cycloprop-2-en-1-yl anion
• cyclobut-2-en-1-yl anion
• cyclopent-2-en-1-yl anion
• cyclohex-2-en-1-yl anion

The preferred systematic names for anions that can be derived by deprotonation are formed according to P-72.2.2.1:

P-72.2.2.1 Anions derived from parent hydrides and their derivatives

An anion derived formally by the removal of one or more hydrons from any position of a neutral parent hydride is preferably named by using the suffix ‘-ide’, with elision of the final letter ‘e’ of the parent hydride, if any. Numerical prefixes ‘di’, ‘tri’, etc. are used to denote multiplicity; locants identify positions of the negative charges.

Therefore, the preferred IUPAC names for the anions that are given in the question are:

• cycloprop-2-en-1-ide
• cyclobut-2-en-1-ide
• cyclopent-2-en-1-ide
• cyclohex-2-en-1-ide

## Cations

In general nomenclature, also cations may be named by using functional class nomenclature; but again, the resulting names are not the preferred IUPAC names.

P-73.2.1 Functional class names

Cationic compounds that can be considered as being derived formally by removal of electrons from the corresponding radical may be named by adding the class name ‘cation’ as a separate word after the name of the radical. Polycations are indicated by adding the multiplying prefixes ‘di’, ‘tri’, etc., as appropriate, to the class name. Systematic names formed by using the suffix ‘ylium’ are preferred IUPAC names (see P-73.2.2). When the charge is not localized, the structure is enclosed in square brackets.

The functional class names for the cations that are given in the question are:

• cycloprop-2-en-1-yl cation
• cyclobut-2-en-1-yl cation
• cyclopent-2-en-1-yl cation
• cyclohex-2-en-1-yl cation

The preferred systematic names for cations that can be derived by removal of a hydride ion are formed according to P-73.2.2.1.2:

P-73.2.2.1.2 General method

According to the general method, cations formally derived by the removal of one hydride ion, $\ce{H-}$, from any position of a parent hydride are named by adding the suffix ‘-ylium’ to the name of the parent hydride, with elision of the final 'e' in the name of the parent hydride, if present. Di- and polycations formally derived by the removal of two or more hydride ions from the parent hydride are named by using the suffix ‘ylium’ and the multiplying prefixes ‘bis’, ‘tris’, etc. (…)

Therefore, the preferred IUPAC names for the cations that are given in the question are:

• cycloprop-2-en-1-ylium
• cyclobut-2-en-1-ylium
• cyclopent-2-en-1-ylium
• cyclohex-2-en-1-ylium

Regarding the anions in the first row of examples, I think rule RC 83.1 (Anionic compounds with anionic centers derived formally by removal of hydrons) apply. Either one uses the "radicofunctional names" (subsection RC 83.1.1):

Anionic compounds that can be considered as being derived formally by addition of electrons to the corresponding radical (see RC-81) may be named by adding the class name "anion" as a separate word after the name of the radical.

which leads to cyclopropenyl anion, cyclobutenyl anion, etc. as in your question. Following RC-83.1.2. "Anionic centers in parent hydrides", and in particular RC-83.1.2.1. "The operational suffix "-ide"." alternatively states

An anionic center derived formally by the removal of one or more hydrons from any position of a neutral parent hydride is named by replacing the final "e" of the name of the parent hydride, if present, by the operational suffix "-ide", or by adding the suffixes "-ide", "-diide", etc., to the name of the parent hydride.

which would lead to a names analogue to cyclopropenide, cyclobutenide, etc.

For the cations presented in the second row, rule RC 82.2 (Cationic compounds with cationic centers derived formally by the loss of hydride ions) is applicable. Again, either the name is radicofunctional (subsectio RB 82.2.1) where

Cationic compounds that can be considered as being derived formally by the removal of electrons from the corresponding radical may be named by adding the class name "cation" as a separate word after the name of the radical (see RC-81). Polycations are indicated by adding the numerical prefixes "di-", "tri-", etc., as appropriate, to the class term.

leads to the used naming of cyclopropenyl cation, cyclobutentyl cation, etc.. Or, RC-82.2.2.2. (Cationic centers in parent hydrides other than those described by RC-82.2.2.1.) which accomodates unsaturated compounds stating that

A cationic center derived formally by the removal of one or more hydride ions from any position of a neutral parent hydride or hydro derivative, except as provided by RC-82.2.2.1, is named by replacing the final "e" of the parent hydride name, if present, by the operational suffix "-ylium", or by adding operational suffixes such as "-bis(ylium)", "-tris(ylium)", etc., to the name of the parent hydride.

which would lead to cyclopropenylium, cyclobutenylium, etc.

• Please note that the links point to obsolete IUPAC recommendations from 1993. However, that does not necessarily mean that names formed according old recommendations are no longer in accordance with current recommendations. – Loong Jul 9 '17 at 7:33
• @Loong Definitely I'll invest time for reading the 2013 edition, including a curious eye of how it evolved over the last 20 years. – Buttonwood Jul 9 '17 at 21:23