Annulenes Aromaticity Shortcut
Table of Content
- Introduction
- Annulenes
- Aromaticity of Annulenes
- The case of [8] Annulene
- The case of [10] Annulene
- The case of bridgehead [10] Annulene
- The case of [12] Annulene
- The case of [14] Annulene
- The case of [16] Annulene
- The case of some higher annulenes
- Preparation of some annulenes
- Summary
Introduction
At this stage of your chemical journey, you have mastered the essential criteria a compound must satisfy to be classified as aromatic or anti-aromatic.
A molecule is aromatic only when all of the following conditions are simultaneously met:
- It is cyclic, planar, and has continuous delocalization of π electrons (electrons in p orbitals) regardless of whether lone pairs, negative charges, or positive charges are involved (i.e., having electrons or a vacant p orbital).
- The delocalized π-electron cloud must contain a total of (4n + 2) π electrons, where n is a whole number (i.e., n = 0, 1, 2, 3, and so on).
Putting n = 0 in (4n + 2) π, we get 2 π electrons; similarly, putting n = 1, we get 6 π electrons; n = 2 gives 10 π electrons; n = 3 gives 14 π electrons, and so on.
In this module, we shall discuss in detail about these classifications for a series of compounds — specifically the fully conjugated monocyclic hydrocarbons — collectively referred to as annulenes.
Annulenes
Annulenes are the completely conjugated monocyclic hydrocarbons containing an even number of carbon atoms. They have the general formula CnHn (when n is an even number) or CnHn + 1 (when n is an odd number). In compact notation, they are expressed as (CH=CH)n.
As per convention, annulenes with 7 or more carbon atoms are named as [n] annulene. That is, they are named by indicating the number of carbon atoms in the ring inside a square bracket before the root word annulene.
e.g., Benzene – [6] annulene, cyclooctatetraene – [8] annulene.
You are already familiar with the first 3 members of the series, [4]-, [6]-, and [8]-annulene, though you may recognize them by the classical names 1,3-cyclobutadiene, benzene, and 1,3,5,7-cyclooctatetraene.
Some other examples of higher annulenes include [14]-annulene (Cyclotetradecaheptaene), [18]-annulene (Cyclooctadecanonaene), and [22]-annulene (Cyclodocosahendeceane).
Aromaticity of Annulenes
Annulenes span the full electronic spectrum: they may be aromatic, anti-aromatic, or non-aromatic.
As key benchmarks: [4] Annulene (cyclobutadiene) is anti-aromatic, [6] Annulene (benzene) is the classic aromatic, while [8] Annulene — i.e., cyclooctatetraene — is non-aromatic. The electronic character of each annulene is elegantly rationalized through Hückel’s rule, which we have already studied in detail for these three cases of annulenes. Let us now take some higher annulenes, progressively broadening our understanding.
The case of [8] Annulene
Cyclooctatetraene can be assumed to have a planar cyclic conjugated system which has 4n π e⁻ where n = 2 as shown in the figure.
This regular planar octagon has bond angles of 135° with large bond angle strain due to large deviation from sp² bond angles of 120°. To overcome this strain, the molecule assumes a distinctive non-planar, tub-shaped conformation with angles C=C−C = 126.1° and C=C−H = 117.6°.
The tub-shaped structure is not planar, not aromatic, and neither anti-aromatic (as non-planarity disrupts continuous electron delocalization). Consequently, the molecule is non-aromatic. Checking the two aromaticity criteria confirms that we classify cyclooctatetraene as non-aromatic, as neither condition is fully satisfied.
8 π electrons total.
Relating this with its reactions, we have the facts to justify its non-aromatic behavior. It behaves like a typical alkene and undergoes addition reactions with electrophilic reagents like Br₂ and HCl, oxidation by KMnO₄, and not substitution reactions like benzene does.
The non-aromatic nature of cyclooctatetraene is further clarified by Molecular Orbital theory, shown below using MO theory. The polygon rule discussed earlier gives the energy levels of various molecular orbitals — revealing three bonding, two non-bonding, and three anti-bonding MOs.
Since the non-bonding π orbitals are half-filled, a perfectly planar model would predict it to be anti-aromatic. However, cyclooctatetraene is not planar, but a tub-shaped molecule. The p-orbitals of one sp² hybridized carbon are not coplanar with those of the neighboring ones; therefore, no effective overlapping between adjacent p-orbitals takes place.
This non-coplanarity completely circumvents anti-aromaticity, leaving the molecule non-aromatic.
The case of [10] Annulene
As noted previously, despite possessing 10 π electrons that seem to satisfy the (4n + 2) π electron requirement of Hückel’s Rule, [10] annulene is in reality non-aromatic, because [10] annulene cannot attain the mandatory planar configuration needed for full conjugation.
If one looks at the structure of this molecule, if we draw the structure as in Figure (a) which appears planar with all double bonds cis, or as in Fig. (b) where one double bond is trans and the other four are cis, in both these situations there is a lot of angular strain within the ring.
Attempting to relieve angular strain by placing two double bonds trans as in Fig. (c), the planarity is lost as the two H atoms facing each other inside the ring cavity create significant steric repulsion against each other. Consequently, the molecule adopts a puckered, non-planar conformation with the two semi-cyclic halves angled relative to each other.
Hence, due to non-bonded interactions between the internal protons, the molecule acquires a non-planar geometry — blocking delocalization and rendering the molecule non-aromatic, though it has 10 e⁻ available for delocalization.
The case of bridgehead [10] Annulene
Remarkably, replacing the two internal H atoms of [10] annulene with a methylene bridge above the ring resolves the strain problem and enables a flat, planar geometry.
Let us consider the case of 9,10-methano[10]annulene. Here, the bridgehead carbons at positions 9 and 10 remain part of the core, but there are no hydrogen atoms creating internal strain. Hence, the 10 carbon atoms remain in a plane, thereby fulfilling the conditions of cyclicity, planarity, and continuous delocalization of π electrons in this 10 π e⁻ system, which is hence aromatic.
Compare this with naphthalene, a 10 π e⁻ system which is aromatic, and focus only on the continuously delocalized 10 π electrons moving around in a cyclic fashion on a planar structure.
Similar to methylene as a bridge on [10] annulene, its oxygen and nitrogen analogs are equally aromatic. For example, 9,10-oxa[10]annulene satisfies cyclicity, planarity, and continuous 10 π electron delocalization, confirming its aromatic identity.
Similarly, 9,10-aza[10]annulene is aromatic, fulfilling the criteria of cyclicity, planarity, and uninterrupted delocalization of 10 π electrons.
The case of [12] Annulene
The structure of [12] Annulene is planar and shown in the figure below. The three internal hydrogen atoms are well separated and impose no steric strain, permitting a planar layout. This makes [12] annulene a cyclic, planar system with unbroken delocalization of π electrons, fulfilling the first condition.
But the number of π electrons continuously delocalized are 12, i.e., 4n π electrons (with n = 3). Being a 4n π e⁻ system, [12] annulene is unambiguously anti-aromatic.
The case of [14] Annulene
This is a 14 π e⁻ system i.e., a (4n + 2) π e⁻ system and can be presumed to be aromatic. However, it fails to undergo substitutive nitration or sulfonation — hallmarks of aromatic character — pointing to non-aromatic behavior.
Let us examine why. As can be seen from the structure, the 'H' atoms present at the interior of the ring interfere with each other, and X-ray analysis shows that the molecule is twisted out of planarity.
Also, it was observed that Dehydro-[14]annulene, formed by the removal of two interfering hydrogens, leads to the formation of an internal triple bond and a planar molecule. The two e⁻ from one of the π bonds of the −C≡C− unit are delocalized into the aromatic π system, and the molecule becomes aromatic.
The other pair of π e⁻ does not interact with the delocalized system as it sits at a right angle to the conjugated system of π e⁻s. Some more examples of stable bridged [14] annulene are shown below:
The case of [16] Annulene
Checking [16] annulene against our two criteria: the first condition is fully met, with the molecule being cyclic, planar, and exhibiting continuous π electron delocalization of π electrons. In the second condition, it is a 16 π electron system — a (4n) π count with n = 4.
Therefore, we conclude that [16] annulene is anti-aromatic. In other words, [16] Annulene is a 4n π e⁻ system, so it is anti-aromatic in nature.
The case of [18] Annulene
[18] Annulene is aromatic in nature. It is a (4n + 2) π e⁻ system when n = 4. It has nine conjugated double bonds. The internal hydrogens do not pose steric conflict, allowing the molecule to be planar for complete delocalization of π e⁻s.
The case of some higher annulenes
In all higher annulenes, the internal hydrogen atoms are sufficiently spaced to eliminate transannular steric repulsion. These molecules are planar and satisfy the first criterion: cyclic architecture, planarity, and continuous π electron delocalization.
In order to classify them, you just check whether it is a (4n + 2) π e⁻ system or a (4n) π e⁻ system, and accordingly, they shall be aromatic or anti-aromatic respectively. Representative examples include:
- [20] annulene: (4n) π e⁻ system where n = 5; hence, anti-aromatic.
- [22] annulene: (4n + 2) π e⁻ system where n = 5; hence, aromatic.
- [24] annulene: (4n) π e⁻ system where n = 6; hence, anti-aromatic.
- [26] annulene: (4n + 2) π e⁻ system where n = 6; hence, aromatic.
Preparation of Some Annulenes
- Preparation of [14] Annulene
The synthesis of [14] annulene is accomplished by oxidative coupling of a polyene in the presence of cupric acetate in pyridine. It results in a cyclic structure containing two alkynyl groups, which on further treatment with potassium t-butoxide in the presence of t-butyl alcohol and reduction with H₂ – Pd/C gives [14] annulene.
- Preparation of [18] Annulene
The following scheme is employed for synthesizing [18] annulene. The key step involves coupling three moles of hexa-1,5-diyne in the presence of cupric acetate in pyridine to give a cyclic trimer (II).
This cyclic trimer undergoes rearrangement in the presence of t-BuOK which gives a 1,7,13-tridehydro[18]annulene (III), which is converted to [18] annulene upon controlled partial hydrogenation.
- Preparation of bridgehead annulene: 9,10-methano[10]annulene
The following scheme represents the synthesis of 9,10-methano[10]annulene.
Summary
Annulenes are the completely conjugated monocyclic hydrocarbons containing an even number of carbon atoms. They have the general formula CnHn (when n is an even number) or CnHn + 1 (when n is an odd number).
The first 3 members of the series are [4]-, [6]-, and [8]-annulene, but you must have known them as 1,3-cyclobutadiene, benzene, and 1,3,5,7-cyclooctatetraene.
Annulenes could be aromatic, anti-aromatic, or non-aromatic.
- [4] Annulene: anti-aromatic
- [6] Annulene: aromatic
- [8] Annulene: non-aromatic.
- [10] Annulene: non-aromatic.
- [12] Annulene: anti-aromatic.
- [14] Annulene: anti-aromatic.
- [18] Annulene: aromatic.
The following bridgehead [10] annulenes are aromatic:
- 9,10-methano[10]annulene
- 9,10-oxa[10]annulene
- 9,10-aza[10]annulene
