Diels–Alder: six electrons in one ring
Follow the concerted [4+2] cycloaddition of butadiene and ethylene, inspect the endo rule, and run the retro reaction backwards.
Goal
Recognise the cyclic 6-electron transition state, why the reaction is stereospecific, and why endo is kinetically preferred.
Apparatus and reagents
The mechanism viewer below; optionally a Woodward–Hoffmann correlation diagram to compare.
Procedure
- Mode 0: send the diene and dienophile through the cyclic transition state; count the three curved arrows.
- Mode 1: compare the endo and exo products; note which substituents sit under the new ring.
- Mode 2: reverse the arrow — heating cyclohexene derivatives can regenerate a diene plus an alkene (retro-Diels–Alder).
- Predict the product stereochemistry when cyclopentadiene adds to maleic anhydride.
What to observe
- No intermediate exists: all three π bonds reorganise at once through a six-membered cyclic transition state with aromatic-like electron circulation.
- Electron-withdrawing groups on the dienophile speed the reaction and end up endo — secondary orbital overlap stabilises the endo transition state.
Explanation
A [4+2] cycloaddition is thermally allowed by the Woodward–Hoffmann rules: 4n+2 electrons moving in a suprafacial–suprafacial fashion pass through a single aromatic-like transition state, so the reaction is concerted and stereospecific (cis substituents stay cis). The endo rule reflects stabilising overlap between the dienophile’s π substituents and the diene’s inner p orbitals. The reverse (retro-Diels–Alder) competes at high temperature — think of the fur cracking of cyclopentadiene dimers.
History of the experiment
Chemists behind it
Virtual experiment: a simplified model to build intuition. It does not replace real lab work or safety training; never repeat chemistry at home without supervision.