Annulenes are monocyclichydrocarbons that contain the maximum number of non-cumulated or conjugated double bonds ('mancude'). They have the general formula CnHn (when n is an even number) or CnHn+1 (when n is an odd number). The IUPAC accepts the use of 'annulene nomenclature' in naming carbocyclic ring systems with 7 or more carbon atoms, using the name '[n]annulene' for the mancude hydrocarbon with ncarbonatoms in its ring,[1] though in certain contexts (e.g., discussions of aromaticity for different ring sizes), smaller rings (n = 3 to 6) can also be informally referred to as annulenes. Using this form of nomenclature 1,3,5,7-cyclooctatetraene is [8]annulene and benzene is [6]annulene (and occasionally referred to as just 'annulene').[2][3]
Structure and AFM image of a hexadehydrotribenzo[12]annulene
The discovery that [18]annulene possesses a number of key properties associated with other aromatic molecules was an important development in the understanding of aromaticity as a chemical concept.
Annulenes may be aromatic (benzene, [6]annulene and [18]annulene), non-aromatic ([8] and [10]annulene), or anti-aromatic (cyclobutadiene, [4]annulene). Cyclobutadiene is the only annulene with considerable antiaromaticity, since planarity is unavoidable. With [8]annulene, the molecule takes on a tub shape that allows it to avoid conjugation of double bonds. [10]Annulene is of the wrong size to achieve a planar structure: in a planar conformation, ring strain due to either steric hindrance of internal hydrogens (when some double bonds are trans) or bond angle distortion (when the double bonds are all cis) is unavoidable. Thus, it does not exhibit appreciable aromaticity.
When the annulene is large enough, [18]annulene for example, there is enough room internally to accommodate hydrogen atoms without significant distortion of bond angles. [18]Annulene possesses several properties that qualify it as aromatic.[5] However, none of the larger annulenes are as stable as benzene, as their reactivity more closely resembles a conjugated polyene than an aromatic hydrocarbon.
In general, charged annulene species of the form [C4n+2+qH4n+2+q]q (n = 0, 1, 2, ..., q = 0, ±1, ±2, 4n + 2 + q ≥ 3) are aromatic, provided a planar conformation can be achieved. For instance, C5H5–, C3H3+, and C8H82– are all known aromatic species.
^Dublin City University Annulenes April 7, 2005, at the Wayback Machine
^Johnson, Suzanne M.; Paul, Iain C.; King, G. S. D. (1970). "[16]Annulene: the crystal and molecular structure". Journal of the Chemical Society B: Physical Organic: 643–649. doi:10.1039/j29700000643. ISSN 0045-6470.
^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.
External linksedit
NIST Chemistry WebBook - [18]annulene
of [14] and [18]annulene
January 01, 1970
annulene, confused, with, aniline, monocyclic, hydrocarbons, that, contain, maximum, number, cumulated, conjugated, double, bonds, mancude, they, have, general, formula, cnhn, when, even, number, cnhn, when, number, iupac, accepts, annulene, nomenclature, nami. Not to be confused with aniline Annulenes are monocyclic hydrocarbons that contain the maximum number of non cumulated or conjugated double bonds mancude They have the general formula CnHn when n is an even number or CnHn 1 when n is an odd number The IUPAC accepts the use of annulene nomenclature in naming carbocyclic ring systems with 7 or more carbon atoms using the name n annulene for the mancude hydrocarbon with n carbon atoms in its ring 1 though in certain contexts e g discussions of aromaticity for different ring sizes smaller rings n 3 to 6 can also be informally referred to as annulenes Using this form of nomenclature 1 3 5 7 cyclooctatetraene is 8 annulene and benzene is 6 annulene and occasionally referred to as just annulene 2 3 Structure and AFM image of a hexadehydrotribenzo 12 annulene The discovery that 18 annulene possesses a number of key properties associated with other aromatic molecules was an important development in the understanding of aromaticity as a chemical concept In the related annulynes one double bond is replaced by a triple bond Contents 1 Aromaticity 2 Gallery 3 See also 4 References 5 External linksAromaticity editAromaticity of the even annulenes n aromaticity 4 antiaromatic 6 aromatic 8 nonaromatic 10 nonaromatic 12 weakly antiaromatic 14 weakly aromatic 16 nonaromatic 4 18 aromatic Annulenes may be aromatic benzene 6 annulene and 18 annulene non aromatic 8 and 10 annulene or anti aromatic cyclobutadiene 4 annulene Cyclobutadiene is the only annulene with considerable antiaromaticity since planarity is unavoidable With 8 annulene the molecule takes on a tub shape that allows it to avoid conjugation of double bonds 10 Annulene is of the wrong size to achieve a planar structure in a planar conformation ring strain due to either steric hindrance of internal hydrogens when some double bonds are trans or bond angle distortion when the double bonds are all cis is unavoidable Thus it does not exhibit appreciable aromaticity When the annulene is large enough 18 annulene for example there is enough room internally to accommodate hydrogen atoms without significant distortion of bond angles 18 Annulene possesses several properties that qualify it as aromatic 5 However none of the larger annulenes are as stable as benzene as their reactivity more closely resembles a conjugated polyene than an aromatic hydrocarbon In general charged annulene species of the form C4n 2 qH4n 2 q q n 0 1 2 q 0 1 2 4n 2 q 3 are aromatic provided a planar conformation can be achieved For instance C5H5 C3H3 and C8H82 are all known aromatic species Gallery edit nbsp Cyclobutadiene 4 annulene nbsp Benzene 6 annulene nbsp Cyclooctatetraene 8 annulene nbsp Cyclododecahexaene 12 annulene nbsp Cyclotetradecaheptaene 14 annulene nbsp Cyclooctadecanonaene 18 annulene nbsp Cyclodocosahendecaene 22 annulene See also edit nbsp Wikimedia Commons has media related to Annulenes Annulyne Circulene FulvenesReferences edit IUPAC Compendium of Chemical Terminology 2nd ed the Gold Book 1997 Online corrected version 2006 annulene doi 10 1351 goldbook A00368 Ege S 1994 Organic Chemistry Structure and Reactivity 3rd ed D C Heath and Company Dublin City University Annulenes Archived April 7 2005 at the Wayback Machine Johnson Suzanne M Paul Iain C King G S D 1970 16 Annulene the crystal and molecular structure Journal of the Chemical Society B Physical Organic 643 649 doi 10 1039 j29700000643 ISSN 0045 6470 Oth Jean F M Bunzli Jean Claude De Julien De Zelicourt 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 External links editNIST Chemistry WebBook 18 annulene Structure of 14 and 18 annulene Retrieved from https en wikipedia org w index php title Annulene amp oldid 1155563174, wikipedia, wiki, book, books, library,
