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 cyclobutadiene, benzene and cyclooctatetreene 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
[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

font face="Arial"[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