6

12 membered rings

[12] annulene

predicted to be antiaromatic if flat

very unstable

14 membered rings

[14] annulene

predicted to be aromatic if planar; but again steric clashes between internal hydrogens
lead to distortion from planarity. Bond lengths 1.35-1.41Å: essentially aromatic.

16 membered rings

[16] annulene

predicted to be antiaromatic if flat. IT exhibits
alternating bond lengths, is non-planar and
therefore non-aromatic.

18 membered rings

[18] annulene

planar bonds, 1.38-1.42Å. Aromatic; no transannular strain, no angle strain.

2.

Aromatic Electrophilic Substitution

pentadienyl cation (arenium ion)

It’s typical of all of the intermediates in electrophilic aromatic substitution.

2.1.

General Reaction Profile

•
•

electrophile (E+) is attracted to electron rich aromatic ring
generates σ-complex

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H+