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

IMAGE aromatics99.gif


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

IMAGE aromatics100.gif
IMAGE aromatics101.gif
IMAGE aromatics102.gif

H

H

IMAGE aromatics103.gif

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

IMAGE aromatics104.gif
IMAGE aromatics105.gif
IMAGE aromatics106.gif

H

H

H+