The Woodward Hoffman description of the Diels-Alder reaction

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The best way to make sense of the Woodward-Hoffmann rules is to try using them.

Click on the structures below to see how this works for the Diels-Alder reaction

.Sn2

‘In allowed thermal pericyclic reactions the number of (4q+2)s and (4r)a components must be odd.’

Let’s break it down:

For allowed pericyclic reactions:

(Number of suprafacial components with 2, 6 or 10 electrons)
+
(Number or antarafacial components with 0, 4 or 8 electrons)
=ODD NUMBER

 

5. Finally add up the components:

Number of (4q+2)s components = 1, Number of (4r)a components = 0

No. of (4q+2)s + No. of (4r)a components = 0 +1 =1 = AN ODD NUMBER

The Diels-Alder reaction is an allowed thermal pericyclic reaction.

R. B. Woodward and R. Hoffmann, Angew. Chemie Int. Ed. English, 1969, 8, 781–853.

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