Cyclooctadecanonaene

Cyclooctadecanonaene or [18]annulene is an organic compound with chemical formula C
18
H
18
. Cyclododecanonaene is an annulene, a cyclic hydrocarbon molecule with formula (CH)n consisting of alternating single and double bonds connecting the carbon atoms. (The single and double bonds may only be formal, because delocalization may cause the bonds to take on true bond orders in between 1 and 2.) Cyclooctadecanonaene is generally regarded as possessing a stabilizing chemical property known as aromaticity, due to its cyclic structure and interacting ring of 18 π electrons above and below the plane of the molecule. This makes the molecule an example of Hückel's rule, derived from molecular orbital theory, which states that a planar, continuous ring of 4n + 2 π electrons, n an integer, possesses aromatic stabilization.

Cyclooctadecanonaene
Identifiers
3D model (JSmol)
ChemSpider
Properties
C18H18
Molar mass 234.342 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Benzene, which can be considered to be [6]annulene (n = 1), exhibits extraordinary chemical stability due to this property and is regarded as the prototypical example of an aromatic molecule. In fact, the term "aromaticity", before being redefined as a concept in chemical bonding, originated as a descriptor for "aromatic smell" of benzene and its many derivatives. The next annulenes predicted to be aromatic are [10]annulene (n = 2) and [14]annulene (n = 3). However, these [4n + 2]annulenes possess interior hydrogen atoms that bump into each other and prevent the molecule from achieving the required planar configuration in an effect known as transannular strain. Consequently, [14]annulene is only marginally aromatic (based on NMR properties), while [10]annulene is considered to be nonaromatic. Certain 10 and 14 π electron systems with structures modified to remove these clashing hydrogens like 1,6-methano[10]annulene, for example, were later discovered to possess aromatic stabilization and helped bolster the validity of the 4n + 2 rule. On the other hand [4n]annulenes do not show any evidence of special stabilization, and when they are geometrically forced to be planar, they actually show evidence of a special destabilization, known as antiaromaticity. Because antiaromaticity is defined to be a destabilizing property, there are very few (if any) unambiguous examples of antiaromatic systems, whereas a vast number of examples of aromatic compounds have been found, resulting in the concept of aromaticity becoming well-accepted and regarded as an important consideration in chemical structure theory.

Importantly, [18]annulene (n = 4) is the first annulene after benzene to possess significant stabilization because it is large enough to comfortably accommodate six hydrogen atoms in its interior. The discovery of aromaticity stabilization for [18]annulene secured the compound an important place in the history of chemical bonding theory by confirming earlier theoretical predictions and is considered to be a triumph for the use of molecular orbital theory in organic chemistry, since simple versions of valence bond theory do not readily explain the 4n + 2 rule.

The usual isomer that [18]annulene refers to is the most stable one, containing six interior hydrogens and twelve exterior ones, with the nine formal double bonds in the cis,trans,trans,cis,trans,trans,cis,trans,trans configuration. It is reported to be a red-brown crystalline solid. The 1H NMR of this compound exhibits the hallmarks of a system with an aromatic ring current, with the 12H signal of the exterior hydrogens at 9.25 ppm, while the 6H signal of the interior hydrogens resonates at a remarkable −2.9 ppm in THF-d8 at −60 °C. On the other hand, a single signal at 5.45 ppm (the weighted average of the two individual signals) is observed at 120 °C. This is consistent with rapid exchange of the exterior and interior hydrogens at that temperature. The bond lengths in [18]annulene are in between those of single and double carbon–carbon bond, with two bond lengths observed crystallographically: 138.9 pm (concave edges) and 140.7 pm (convex edges). These bond lengths are indicative of significant delocalization. The favorability of delocalization is, in turn, interpreted as evidence for aromaticity. For comparison, these values are close to the bond length of benzene (140 pm).[1]

Based on the enthalpy of hydrogenation, the overall resonance energy has been estimated to be 37 kcal/mol.[2] This is about the same as that of benzene; however, this energy is spread out over 18 atoms instead of 6, so [18]annulene experiences a weaker stabilization than benzene. In terms of reactivity, it is somewhat more air- and light-stable than [14]annulene and [10]annulene, which are, respectively, weakly aromatic and nonaromatic due to transannular interactions. Nevertheless, it rapidly undergoes addition reactions with electrophiles, much like other polyenes, and attempts to effect Friedel-Crafts-type reactions on [18]annulene failed.[3]

Despite the usual interpretation of [18]annulene as an 18-electron aromatic system, one recent theoretical studies suggests that [18]annulene may be thought of as having only three completely delocalized π bonds associated with its aromaticity, while the other six π bonds represent conjugated three-center-two-electron ("3c-2e") π bonds on the periphery of the molecule.[4]

Synthesis

The compound was first synthesised by Franz Sondheimer.[5] The original synthesis started by the Eglinton reaction of the di-alkyne 1,5-hexadiyne with copper(II) acetate in pyridine to give the trimer, followed by deprotonation and isomerization with potassium tert-butoxide in tert-butanol and was concluded with hydrogen organic reduction with the Lindlar catalyst.[6]

See also

References

  1. Jux, NorbertÂ; R. Schleyer, Paul v; Majetich, GeorgeÂ; Meyer, KarstenÂ; Hampel, FrankÂ; W. Heinemann, Frank; V. Nizovtsev, Alexey; Lungerich, Dominik (2016). "[18]Annulene put into a new perspective". Chemical Communications. 52 (25): 4710–4713. doi:10.1039/C6CC01309K. PMID 26953607.
  2. Oth, Jean F. M.; Bünzli, Jean-Claude; De Julien De Zélicourt, Yves (1974-11-06). "The Stabilization Energy of [18] Annulene. A thermochemical determination". Helvetica Chimica Acta. 57 (7): 2276–2288. doi:10.1002/hlca.19740570745. ISSN 0018-019X.
  3. Sondheimer, F., Wolovsky, R. and Amiel, Y. (1962). "Unsaturated Macrocyclic Compounds. XXIII. The Synthesis of the Fully Conjugated Macrocyclic Polyenes Cyclooctadecanonaene ([18]Annulene), Cyclotetracosadodecaene ([24]Annulene), and Cyclotriacontapentadecaene ([30]Annulene)". J. Am. Chem. Soc. 68 (2): 274–284. doi:10.1021/ja00861a030.CS1 maint: multiple names: authors list (link)
  4. Ivanov, A.; Boldyrev. A (2014). "Deciphering aromaticity in porphyrinoids via adaptive natural density partitioning". Org. Biomol. Chem. 12 (32): 6145–6150. doi:10.1039/C4OB01018C.
  5. In the literature and some internet references, Sondheimer is sometimes misspelled as Sandheimer.
  6. K. Stöckel and F. Sondheimer (1988). "[18]Annulene". Organic Syntheses.; Collective Volume, 6, p. 68
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