Annulenes aremonocyclichydrocarbons that contain the maximum number of non-cumulated or conjugated double bonds ('mancude'), and their derivatives. They have the general formula C_nH_n (whenn is an even number) or C_nH_n+1 (whenn is an odd number). TheIUPAC 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 withncarbonatoms 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]

The discovery that [18]annulene possesses a number of key properties associated with other aromatic molecules was an important development in the understanding ofaromaticity as a chemical concept.

In the relatedannulynes, one double bond is replaced by atriple bond.

Annulenes may bearomatic ([6]annulene (benzene) and [18]annulene),non-aromatic ([8] and [10]annulene), oranti-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 eithersteric hindrance of internal hydrogens (when some double bonds aretrans) or bond angle distortion (when the double bonds are allcis) 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[C_4n+2+qH_4n+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,C₅H−5,C₃H+3, andC₈H2−8 are all known aromatic species.

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  Cyclobutadiene ([4]annulene)
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  Benzene ([6]annulene)
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  Cyclooctatetraene ([8]annulene)
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  Cyclodecapentaene ([10]annulene)
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  Cyclododecahexaene ([12]annulene)
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  Cyclotetradecaheptaene ([14]annulene)
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  Cyclooctadecanonaene ([18]annulene)
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  Cyclodocosahendecaene ([22]annulene)

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• Annulyne
• Circulene
• Fulvenes

• NIST Chemistry WebBook - [18]annulene
• Structure of [14] and [18]annulene

• Annulenes
• Physical organic chemistry

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