4

1.6.

The Hückel Rule at Work

3 membered rings

∆H = 153 kcal/mol

∆H = 173 kcal/mol

at least 20 kcal/moleasier before we take ring strain into account.

4 membered rings

[4] annulene
antiaromatic 4π electrons, unstable > 4K

[6] annulene

cyclobutadienyl dication

5 membered rings

cyclopentadienyl anion

cyclopentadiene is relatively acidic for a hydrocarbon; pKa ~ 16

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Cl

Cl

Cl

SbCl5

SbCl-
6

ClO-
4

AgBF4

BF-
4
+

AgCl

+

+

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Cl

Cl

SbF5

SO2

-75oC

2π e-

2 SbF5Cl-

2

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