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Organic hydrocarbons that have molecules containing simple single rings of carbon atoms linked by alternating single and double bonds. Such compounds have even numbers of carbon atoms. Cyclo-octatetraene, C8H8, is the next in the series following benzene. Higher annulenes are usually referred to by the number of carbon atoms in the ring, as in [10]-annulene, C10H10, [12]-annulene, C12H12, etc. 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. Thus, [10]-annulene has 4n+2 pi electrons with n=2, but is not aromatic because it is not planar. [14]-annulene also has a suitable number of pi electrons to be aromatic (n=3) but is not planar because of interaction between the inner hydrogens.

The compound [18]-annulene is large enough to be planar and obeys the Hückel rule (4n+2=18, with n=4). It is a brownish red fairly stable reactive solid. NMR evidence shows that it has aromatic character. The annulene with n=7, [30]-annulene, can also exist in a planar form but is highly unstable. See also pseudoaromatic.


Subjects: Chemistry.

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