Mancude rings and annulenes

What do the structures (a), (b) and (c) have in common?

(a)	(b)	(c)

[18]annulene cyclooctadeca-1,3,5,7,9,11,13,15,17-nonaene (PIN) 1,3,5,2,4,6-triazatriphosphinine thiophene

Well, it is obvious that they all are rings. Also, apart from hydrogens in (a) and (c), they have no side chains. Otherwise, they are quite different. The structure (a) is a hydrocarbon. The ring (b) is purely inorganic while (c) is an organic heterocycle. What else?

You can see that in all these structures single bonds alternate with double bonds. Ring systems like this are referred to as mancude, which is an abbreviation of the “maximum number of non-cumulative double bonds”.

Let’s have a look at the structure (a) known as [18]annulene. The term annulene, from Latin annulus “ring” and ‘ene’ for double bond, was coined in the early 1960s by Franz Sondheimer and Reuven Wolovsky [1, 2]. Gold Book defines annulenes as

Mancude monocyclic hydrocarbons without side chains of the general formula C_nH_n (n is an even number) or C_nH_n+1 (n is an odd number).

Order	Formula	Structure	PIN	Annulene name	Group formula	Group name
3	C3H4		cyclopropene	[3]annulene	–C3H3	cycloprop-1-en-1-yl cycloprop-2-en-1-yl
4	C4H4		cyclobuta-1,3-diene	[4]annulene	–C4H3	cyclobuta-1,3-dienyl
5	C5H6		cyclopenta-1,3-diene	[5]annulene	–C5H5	cyclopenta-1,3-dien-1-yl cyclopenta-2,4-dien-1-yl
6	C6H6		benzene	[6]annulene	–C6H5	phenyl
7	C7H8		cyclohepta-1,3,5-triene	[7]annulene	–C7H7	cyclohepta-1,3,5-trien-1-yl cyclohepta-2,4,6-trien-1-yl
8	C8H8		cycloocta-1,3,5,7-tetraene	[8]annulene	–C8H7	cycloocta-1,3,5,7-tetraenyl

And so on.

As the rings grow in size, or rather as the number of double bonds in the ring increases, the cycloalkane-based systematic names progressively become more and more cumbersome. One can see that the name like cycloocta-1,3,5,7-tetraene, while easily analysable as “cyclic hydrocarbon containing eight carbon atoms with four double bonds at positions 1, 3, 5 and 7”, fails miserably to convey its most conspicuous structural feature, that is, its fourfold symmetry. It is so much better to name a ring with n carbon atoms [n]annulene. Even though Gold Book reserves this method for rings with n≥7, I don’t see why we can’t use it for lower annulenes.

The general method of naming substituent groups (other than derived from n-alkanes and cycloalkanes) requires to cite the attachment point locant number, including ‘1’ [3, p. 204]. This makes sense for monovalent groups derived from odd-numbered annulenes. There are two types of attachment points, viz. (I) sp² carbon (>C=) and (II) sp³ carbon (>CH–); since both are assigned the locant number ‘1’, renumbering of locants for double bonds may be required. For example, the groups derived from cyclopenta-1,3-diene are named cyclopenta-1,3-dien-1-yl and cyclopenta-2,4-dien-1-yl (not cyclopenta-1,3-dien-2-yl), for type I and II attachment points, respectively. But for monovalent groups derived from even-numbered annulenes there is no real need to cite the attachment point locant since all attachment points are of the type I. For instance, the name cyclobuta-1,3-dienyl is completely unambiguous, so no need to call it cyclobuta-1,3-dien-1-yl. Even better, we can use ‘[n]annulenyl’ to name even-numbered groups^*.

Now [6]annulene is a special case. Not only its PIN is ‘benzene’ (not cyclohexa-1,3,5-triene), but also the derived monovalent group is called phenyl (not benzenyl). This, you may recall, is an example of suppletion. Benzene is so far the only chemical structure to be assigned the Unicode symbol — in fact, two Unicode symbols: the Kekulé structure ⌬ and delocalised ⏣.

There are three possible divalent groups derived from benzene:

(d)	(e)	(f)

1,2-phenylene 1,3-phenylene 1,4-phenylene

Benzene is the archetypical aromatic molecule. According to Wikipedia,

The first known use of the word “aromatic” as a chemical term — namely, to apply to compounds that contain the phenyl group — occurs in an article by August Wilhelm Hofmann in 1855.

Yes, that same Hofmann who named alkanes practically as we know them now. Curiously, in his paper [4] he mentions “aromatic acids”^† as if the term needed no explanation whatsoever.

Not all annulenes are aromatic. Apart from benzene, [14]-, [18]-, and [22]annulenes are shown to be aromatic, while [4]annulene is antiaromatic. On the other hand, both cyclopentadienide (g) and tropylium (h) ions have six π-electrons and thus are isoelectronic with benzene. Their respective PINs, however, slavishly describe the structures in terms of localised bonds and charges. On top of that, the carbon atom that formally acquires a charge is assigned the locant number ‘1’, thus prompting renumbering of double bond locants: cyclopenta-1,3-diene → cyclopenta-2,4-dienide; cyclohepta-1,3,5-triene → cyclohepta-2,4,6-trienylium. The alternative names [5]annulenide and [7]annulenium are much shorter.

(g)	(h)

C5H5− cyclopenta-2,4-dienide (PIN) [5]annulenide C7H7+ cyclohepta-2,4,6-trienylium (PIN) tropylium (trivial) [7]annulenium

Gold Book defines arenes as

Monocyclic and polycyclic aromatic hydrocarbons.

Benzene is often thought of as the simplest arene, although the simpler cyclopropenylium C₃H₃⁺ and cyclopentadienide C₅H₅⁻ ions are also aromatic hydrocarbons. In any case, benzene is the simplest (neutral) arene molecule.

In higher annulenes, an additional problem arises as there could be quite a number of cis/trans isomers. For instance, the variants on [18]annulene (a) theme include structures (g) and (h), with accordingly unwieldy systematic names.

(g)	(h)

(1E,3Z,5E,7E,9Z,11E,13E,15Z,17E)-cyclooctadeca-1,3,5,7,9,11,13,15,17-nonaene (1Z,3E,5E,7E,9Z,11Z,13E,15E,17E)-cyclooctadeca-1,3,5,7,9,11,13,15,17-nonaene

*	We also can use ‘[n]annulenyl’ for odd-numbered groups if we can’t (or don’t want to) specify attachment points.
†	Hofmann’s list of “monobasic aromatic acids” [4] includes benzoic acid, toluylic acid (now toluic acid) and cuminic acid (now cumic acid), while “bibasic” ones include phthalic acid, terephthalic acid and the titular “insolinic acid” which could be either methylterephthalic acid, 5-methylisophthalic acid (uvitic acid) or 4-methylphthalic acid.

References

1. Sondheimer, F. and Wolovsky, R. (1962) Unsaturated macrocyclic compounds. XXI. The synthesis of a series of fully conjugated macrocyclic polyene-polyynes (dehydro-annulenes) from 1,5-hexadiyne. Journal of the American Chemical Society 84, 260—269.
2. Sondheimer, F. (1971) Recent progress in the annulene field. Pure and Applied Chemistry 28, 331—354.
3. Connelly, N.G., Hartshorn R.M., Damhus, T. and Hutton, A.T. Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005. Royal Society of Chemistry, Cambridge, 2005.
4. Hofmann, A.W. (1856) On insolinic acid. Proceedings of the Royal Society VIII, 1—3.