The Woodward Hoffman description of the Diels-Alder reaction

The Woodward-Hoffman rule

'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)

The best way to make sense of the Woodward-Hoffmann rule is to try using it. Click on the numbers and structures below to see how this works for the Diels-Alder reaction.

Background Colour:


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.