Ch 11 : Annulenes

Chapter 11 : Arenes and Aromaticity

Annulenes

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 1,3-cyclobutadiene, benzene and 1,3,5,7-cyclooctatetraene respectively.
Higher members of the annulene series have also been synthesised and investigated in relation their aromaticity.

Remember that according to the Huckel rule, (4n + 2) π electrons systems are expected to aromatic whereas 4n π electrons systems are not.

[10]- and [14]-annulene are not particularly stable due to geometric factors (look at the JSMOL images below).
[16]-annulene is non-planar, with alternating C=C (average 134 pm) and C-C (average 146 pm) like a non-aromatic polyene.


[10]-annulene	[14]-annulene	[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 H₂and Br₂, and a Diels-Alder with maleic anhydride.

QUESTION How do [10]-, [14]- and [18]-annulene relate to the Huckel rule ? ANSWER