- Annulene
Annulenes are completely conjugated monocyclic
hydrocarbon s. They have the general formula CnHn (n is an even number) or CnHn+1 (n is an odd number). The IUPAC naming conventions are that annulenes with 7 or more carbon atoms are named as ["n"] annulene, where "n" is the number ofcarbon atom s in their ring, [GoldBookRef | title = annulene | file = A00368] though sometimes the smaller annulenes are referred to using the same notation, and benzene is sometimes referred to simply as annulene [Ege, S. (1994) "Organic Chemistry:Structure and Reactivity" 3rd ed.D.C. Heath and Company ] [Dublin City University " [http://www.dcu.ie/~chemist/pratt/annulene/annulene.htm Annulenes] "] .The first three annulenes are
cyclobutadiene ,benzene , andcyclooctatetraene ( [8] annulene). Some annulenes, namely cyclobutadiene,cyclodecapentaene or [10] annulene,cyclododecahexaene or [12] annulene and [14] annulene, are unstable, with cyclobutadiene extremely so.Annulenes may be "
aromatic " (benzene, [14] annulene,cyclooctadecanonaene or [18] annulene), "non-aromatic" ( [8] annulene), or "anti-aromatic" (cyclobutadiene). Only cyclobutadiene and benzene are fullyplanar , though [14] and [18] annulene with all trans double bonds (placing the hydrogens inside the ring) can achieve the planar conformation needed for aromaticity, with [14] and [18] annulene followingHückel's rule with 4n+2 π electrons. [14] annulene does exhibit somering strain due tosteric hindrance .Many of the larger annulenes, [18] annuelene for example, are large enough to minimize the van der Waals strain of internal hydrogens and thermodynamically qualify as aromatic. However, none of the larger annulenes are as stable as benzene, as their reactivity more closely resemebles a conjugated polyene than an aromatic hydrocarbon.
In
annulyne s, one double bond is replaced by analkyne bond.References
External links
* [http://webbook.nist.gov/cgi/cbook.cgi?ID=C2040735&Units=SI NIST Chemistry WebBook] - [18] annulene
* [http://www.ccc.uga.edu/~cwannere/big.pdf Structure] of [14] and [18] annulene
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