Bicalicene is polycyclic

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...

with

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

C₁₆H₈, composed of two

cyclopentadiene Cyclopentadiene is an organic compound with the chemical formula, formula C5H6.LeRoy H. Scharpen and Victor W. Laurie (1965): "Structure of cyclopentadiene". ''The Journal of Chemical Physics'', volume 43, issue 8, pages 2765-2766. It is often ab ...

and two

cyclopropene Cyclopropene is an organic compound with the formula . It is the simplest cycloalkene. Because the ring is highly strained, cyclopropene is difficult to prepare and highly reactive. This colorless gas has been the subject for many fundamental st ...

rings linked into a larger eight-membered ring. There are two isomers: ''cis''-bicalicene and ''trans''-bicalicene. It is a dimer of

calicene Calicene or triapentafulvalene is a hydrocarbon of the fulvalene class with chemical formula C8H6, composed of a cyclopentadiene ring and a cyclopropene ring linked by a double bond. Its name is derived from the Latin ''calix'' meaning "goblet", ...

Synthesis

Bicalicene is prepared by treatment of 1,2-bis(''tert''-butylthio)-3,3-dichlorocyclopropene with cyclopentadiene anion, followed by desulfurizing stannylation with

tributyltin hydride Tributyltin hydride is an organotin compound with the formula (C4H9)3SnH. It is a colorless liquid that is soluble in organic solvents. The compound is used as a source of hydrogen atoms in organic synthesis. Synthesis and characterization The ...

, and then treatment with silica gel.

Properties

''trans''-Bicalicene is

polycyclic aromatic hydrocarbon A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...

, which is unusual for a 16 π electron ring system. Viewed as a unified ring structure,

Hückel's rule In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4''n'' + 2 π electrons, where ''n'' is a non-negative integer. The quantum mechanical basis for its formulation was fir ...

predicts it would be

anti-aromatic Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i.e., it is less stable due to the presence of 4n delocalised (π or lone pair) electrons in it, as opposed to aromaticity. Unlike aroma ...

(4''n'' π electrons). Instead, however, the structure has a dominant partially-delocalized charge-separated structure consisting of four independently-

aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

(4''n''+2 π electron) rings: two as

cyclopropenyl cation The cyclopropenium ion is the cation with the formula . It has attracted attention as the smallest example of an aromatic cation. Its salts have been isolated, and many derivatives have been characterized by X-ray crystallography. The cation and so ...

s (two π electrons each) and two as

cyclopentadienyl anion In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of and abbreviated as Cp−. It is formed from the deprotonation of the molecule cyclopentadiene. Properties The cyclopentadienyl anion i ...

s (six π electrons each). ''cis''-Bicalicene, by contrast, is an antiaromatic hydrocarbon. A resonance structure with four aromatic rings, analogous to the one that makes the ''trans'' isomer stable, would suffer from destabilizing charge effects, and other resonance structures have 4''n'' rather than 4''n''+2 π electrons in at least one ring.

References

{{PAHs Polycyclic aromatic hydrocarbons Cyclopropenes Non-benzenoid aromatic carbocycles Antiaromatic compounds B