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