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Diels Alder Reaction – 1

Every Diels Alder reaction you’ll ever see follows the same pattern.

You’re always breaking three pi bonds and forming two sigma (single) bonds and a pi bond. 

There’s three curved arrows. In the top example below, we’re  breaking 5-6, 1-2, and 3-4, and forming 1-5, 2-3, and 4-6.

No matter what extra groups are on the diene or dienophile, it’s still the same pattern. Always.

One diene that confuses students a lot is cyclopentadiene (It’s the diene inside the five membered ring).

But if you look at the pattern of bonds formed and broken it’s the same pattern as that above:

Break 5-6, 1-2, and 3-4, and form 1-5, 2-3, and 4-6.
Nothing else changes! The C1-C7 bond and the C4-C7 bonds remain exactly the same in both the starting material and product (a reminder: only break and form the bonds that the arrows “tell” you to form and break!).

You might find it helpful to always draw a “bare-bones” Diels-Alder first (with arrows) and then “decorate” it with the substituents later. Numbering the carbons really helps a lot here.

One last note – the Diels Alder of cyclopentadiene can be drawn in multiple ways, either the “top” view or the “side view”. Molecules are just like any other 3-D object; when you’re drawing them, sometimes a change of perspective helps. The product here is a bicyclic compound: 2.2.1 bicycloheptene.

Thanks for reading! James

P.S. From the Reaction Guide: The Diels Alder reaction
P.P.S. Relevant post: Putting Diels Alder Products in Perspective