The prototype: 1,3-butadiene + ethylene, on heating, gives cyclohexene.
1. The Diels–Alder Builds a Six-Membered Ring From a Diene and a Dienophile
A conjugated diene supplies four carbons and a dienophile the other two, closing a cyclohexene ring as three π bonds become two σ bonds plus one alkene.
2. Four Pi Electrons Plus Two Makes It a [4+2] Cycloaddition
The diene brings four π electrons and the dienophile two, so six electrons flow around one loop — a pericyclic, thermally allowed process with no intermediate or catalyst.
3. The Diene Must Be in the s-cis Conformation
Only in the s-cis conformation do both terminal carbons point the same way, close enough to reach both ends of the dienophile at once — an s-trans diene cannot close the ring, which is why locked-s-cis cyclopentadiene is so reactive.
4. Electron-Poor Dienophiles React Fastest
An electron-withdrawing group (carbonyl, ester, nitrile, nitro) lowers the dienophile's LUMO toward the diene's HOMO, so sluggish ethylene becomes reactive as acrolein or maleic anhydride — the "normal electron demand" case.
5. The Reaction Is Concerted, So It Is Stereospecific
All bonds form at once through one cyclic transition state (syn, suprafacial), so the dienophile's geometry is preserved — cis stays cis, trans stays trans — giving a single predictable diastereomer.
1,3-Butadiene + acrolein → a substituted cyclohexene bearing the aldehyde.
6. The Endo Product Is Preferred (the Endo Rule)
In the endo approach the EWG tucks under the diene, where secondary orbital overlap stabilizes the transition state, so the endo product forms faster and predominates as the kinetic product.
7. Summary
Concerted [4+2] cycloaddition · diene + dienophile → cyclohexene · diene must be s-cis · electron-rich diene + electron-poor dienophile · stereospecific (cis stays cis) · endo favored.
Worked example
- This is a [4+2] cycloaddition: the 4π diene (s-cis) plus the 2π dienophile.
- Three π bonds become two new σ bonds and one π bond — one concerted, suprafacial step.
- The product is a six-membered ring containing a single double bond.
Answer. Cyclohexene.
Quiz yourself
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Only in the s-cis conformation do the two terminal carbons of the diene point the same way, close enough to bond to both ends of the dienophile simultaneously. An s-trans diene has its reactive ends pointing in opposite directions and cannot close the six-membered ring.
Electron-withdrawing groups (EWGs) such as carbonyls, esters, nitriles, and nitro groups. They lower the dienophile's LUMO, bringing it closer in energy to the diene's HOMO. Stronger HOMO–LUMO overlap means a faster reaction — this is the "normal electron demand" Diels–Alder.
Because the mechanism is concerted and both new σ bonds form on the same face at once (syn addition), the geometry of the dienophile is retained: a cis-substituted alkene gives a cis product and a trans alkene gives a trans product. Each geometric isomer leads to one specific diastereomer.
It predicts that when the dienophile bears an EWG, the endo product forms preferentially. In the endo transition state the EWG tucks under the diene, gaining stabilizing secondary orbital overlap. That lowers the activation energy, so endo is the faster-forming kinetic product even though the exo isomer is often more thermodynamically stable.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.