5

6 membered rings

Benzene

6π electrons i.e. aromatic

7 membered rings

tropylium cation

6π electrons, predicted to be aromatic

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It has ionic character (mp> 200oC)
It is soluble in water
It is non-soluble in non-polar solvent
It is planar; all bond lengths are equal
It precipitates with AgNO3

It is therefore better represented as:

8 membered rings

[8] annulene: if planar to be predicted by Hückel’s rule to be antiaromatic and
therefore unstable.

However, it does not exist in a planar form but in a “tub”-like shape instead.
It reacts with KMnO4and Br2yielding addition (not substitution) products.

It exists in two double bond isomers

but we can generate an aromatic system by adding 2π electrons:

aromatic, flat: all bonds are of equal length.

10 membered rings

[10] annulene

if flat, then it is expected to be aromatic. Because of severe non-bonded steric
repulsions (transannular strain) it adopts instead an obviously non-planar
conformation. Naphthalene on the other hand is planar and aromatic!

Br

H

Lewis Acid

2 Na

+

2

2 Na+

+

H

H