Which annulenes are planar

The other electron pair colored black behaves similarly to the electron pair in pyridine. Monocyclic compounds made up of alternating conjugated double bonds are called annulenes. Benzene and 1,3,5,7-cyclooctatetraene are examples of annulenes; they are named [6]annulene and [8]annulene respectively, according to a general nomenclature system in which the number of pi-electrons in an annulene is designated by a number in brackets.

Some annulenes are aromatic e. As shown in the following diagram, 1,3,5,7,9-cyclodecapentaene fails to adopt a planar conformation, either in the all cis-configuration or in its 1,5-trans-isomeric form.

The transannular hydrogen crowding that destabilizes the latter may be eliminated by replacing the interior hydrogens with a bond or a short bridge colored magenta in the diagram. Naphthalene and azulene are [10]annulene analogs stabilized by a transannular bond. The bridged [14]annulene compound on the far right, also has aromatic properties. A synthesis of barrelene bicyclo[2. Zimmerman Wisconsin , using a double Hofmann elimination.

As shown in the following diagram, the chemical behavior of this triene confirmed it was not aromatic in the accepted sense of this term. Bromine addition took place rapidly with transannular bond formation, in the same fashion as with norbornadiene bicyclo[2. Pyrolysis of barrelene gave the expected cycloreversion products benzene and acetylene. The heat of hydrogenation of barrelene reflects its thermodynamic stability. Furthermore, the first double bond of barrelene is reduced with the release of An explanation for the lack of aromatic behavior in the case of barrelene may be found by comparing the orbital symmetry of the six component p-orbitals with those of benzene.

Benzene is an annulene in which all six p-orbitals may be oriented with congruent overlapping phases. The cylindrical array of p-orbitals in barrelene cannot be so arranged, as shown in the diagram on the right. Altmetric -. Citations 9. Supporting Information. Cited By. This article is cited by 9 publications. The Journal of Organic Chemistry , 86 19 , Kroeger , Amir Karton.

Dithieno[3,4- b :3',4'- d ]arsole: A Novel Class of Hetero[5]radialenes. European Journal of Organic Chemistry , 26 , Chemistry — A European Journal , 26 12 , Journal of Synthetic Organic Chemistry, Japan , 78 2 , ChemPlusChem , 84 6 , The lower members are not stable as a result of the interactions between hydrogen atoms inside the ring.

This is true even for molecules that have the necessary number of pi electrons to be aromatic compounds. It is a brownish red fairly stable reactive solid. NMR evidence shows that it has aromatic character.

See also pseudoaromatic. From: annulenes in A Dictionary of Chemistry ». Subjects: Science and technology — Chemistry. View all related items in Oxford Reference ». Search for: 'annulenes' in Oxford Reference ». All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single entry from a reference work in OR for personal use for details see Privacy Policy and Legal Notice.