Annulenes are the family of completely conjugated, monocyclic hydrocarbons.
The annulenes are named as [n]-annulene where n is an even number that
represents the number of C atoms in the ring.
We have already met the first 3 members of the series, [4]-, [6]-, and
[8]-annulene but we called them cyclobutadiene, benzene and cyclooctatetreene
respectively.
Higher members of the annulene series have also been synthesised and investigated
in relation to their aromaticity.
Remember that according to the Huckel
rule, (4n + 2) p electrons systems are expected
to aromatic whereas 4n p electrons systems are
not.
[10]- and [14]-annulene are not particularly stable due to geometric factors
[16]-annulene is non-planar, with alternating C=C (average 134 pm)
and C-C (average 146 pm) like a non-aromatic polyene.
[16]-annulene
[18]-annulene
[18]-annulene is almost planar, with CC bonds between 137-143 pm
and an estimated resonance energy of 418kJ/mol
(100kcal/mol) suggesting some aromatic
stability. However, it reacts more like a polyene than benzene :
it undergoes addition reaction with H2 and Br2, and
a Diels-Alder with maleic anhydride.
Question How do [10]-, [14]-
and [18]-annulene relate to the Huckel rule ?
