Pericyclic Reactions: The Diels-Alder and Orbital Symmetry
Some reactions have no intermediate at all. Bonds break and form together in a ring of moving electrons, and whether that is allowed depends on whether the orbitals meeting each other have matching symmetry.
What a learner can do afterwards
- Draws a Diels-Alder reaction with the diene in the required conformation and predicts the ring formed
- Explains the effect of electron-donating and electron-withdrawing substituents on the rate
- Predicts the stereochemistry of the adduct, including the endo preference, and explains where it comes from
- Uses the symmetry of the interacting orbitals to say why a stated cycloaddition is allowed thermally but not photochemically
1 · Read
Pericyclic reactions form their own family with no ions and no radicals, just a ring of moving electrons. The Diels Alder is the flagship: a diene plus a dienophile give a cyclohexene in a single step. Spot the six membered ring with one double bond and you have found the fingerprint.
Shape controls everything because only the s-cis diene can touch both ends of the partner at once. A diene locked in the s-trans shape cannot react at all, while one that can fold into s-cis reacts. Conformation feeds straight into rate.
Substituents tune the meeting of the diene HOMO with the dienophile LUMO. Donating groups on the diene plus withdrawing groups on the dienophile narrow that gap and speed a normal demand reaction. Since both new sigma bonds form together with no intermediate, the dienophile geometry carries over exactly: cis stays cis and trans stays trans.
Two finer points complete the picture. The endo rule says dienophile substituents tuck under the diene pi system, steadied by extra orbital overlap even when the exo product is stabler. And orbital symmetry explains the heating rule for the taught cycloadditions: the six pi electron 4+2 moving suprafacially on both partners is allowed with heat, while a 2+2 of two alkenes would need one alkene antarafacial and is not observed, so it needs light instead.
One concerted step joins diene to dienophile, shape and substituents set the pace, and geometry is preserved.
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