Diels–Alder: push the electrons yourself
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You just pushed every arrow of the Diels–Alder.
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Organic Chemistry 2 · Free interactive demo
Butadiene and dimethyl acetylenedicarboxylate meet and, in one concerted step, six electrons move around a ring: two new sigma bonds form at the diene ends, the diene’s remaining pi bond shifts to the middle, and the alkyne keeps one of its two pi bonds.
Draw all three arrows above in one step. Each arrow is checked as you draw it, and Hint reveals the next move a little at a time.
Turn your own notes into exercises like this one
With the Arrow Lab student plan you can snap a photo of a reaction from your lecture notes or textbook. Arrow Lab reads the starting materials and reagents, you check that reading, and it builds the mechanism as an exercise like the one above: you draw each arrow, get hints when you are stuck, and see right away whether each move is correct.
Scan Standard, Scan Lite and Scan each include monthly scan credits, up to 20 a day. A credit is a unit of work, not a problem: a typical scan uses about 1 credit, working backwards from a product about 2, and a long or tricky mechanism 2–4.
The Diels–Alder mechanism, step by step
The Diels–Alder reaction is the workhorse six-membered-ring synthesis: a conjugated diene in its s-cis shape plus an electron-poor dienophile, no catalyst, no intermediates. With an alkyne as the dienophile the product is a 1,4-cyclohexadiene rather than a cyclohexene, so counting the pi bonds that remain is the whole point.
The 2 steps of the demo, in the exercise's own words. Try drawing each one above before you read it.
Close the six-membered ring
All three arrows act together to close the ring. Each end of the diene bonds to one dienophile carbon; the middle π bond forms between the two inner diene carbons. No ionic intermediate forms.
Products
The alkyne becomes a double bond, not a single bond. The diene provides the other C=C bond. Two new single bonds close the six-membered ring, and no charges form.
How the mechanism is drawn
Heat butadiene with dimethyl acetylenedicarboxylate for a Diels–Alder reaction. Keep the CO₂Me groups unchanged. Each end of the diene bonds to one alkyne carbon; the middle π bond forms between the two inner diene carbons.