Diels Alder 3: Regiochemistry

[@julietifyjulietify6010]

Thanks so much, this really explained it well

[@aniluaPzurC13]

what happened to the double bond that resonated up on the dienophile? Why is it not on the product?

[@aniluaPzurC13]

Also, I don't see how carbons 2 and 3 gets a double bond on the last example ?

[@sunshine-qw5md]

@@aniluaPzurC13 (You probably don't need to know this any more since it's been 2 years, but here's an explanation in case someone else watching this needs help:)
The arrow pushing mechanism at 1:18 should help w understanding this. You can see that one of the pi bonds on the diene moves in between carbon 2 and 3, while the other pi bond's electrons go to form a sigma bond with the dienophile. The dienophile's electrons in the pi bond are also used to form a sigma bond with the diene on the other carbon .
So in the diene, when the electrons in one of the pi bonds is used to form a sigma bond with the dienophile, This turns the carbon from sp2 into sp3. Since it's sp3, it can no longer form a double bond (has no p orbitals), which is why there's only a pi bond in between carbon 2 and 3 in the final structure.
This is the same for any sp3 carbon. If a carbon is sp3 (bonded to four atoms) it can't form a double bond.