The Diels-Alder reaction is a concerted, single-step [4+2] cycloaddition: a conjugated diene (4 π electrons, s-cis conformation) reacts suprafacially with a dienophile (2 π electrons) to build a cyclohexene ring. Two new σ bonds (C1–C6 and C4–C5) form simultaneously in one pericyclic transition state — there is no discrete carbocation or radical intermediate, unlike stepwise SN1/addition mechanisms.
Diene (HOMO) + Dienophile (LUMO) → [4+2] TS → Cyclohexene
Rate: k = A · exp(−Ea / R·T) R = 1.987×10⁻³ kcal/(mol·K)
- Progress slider / Play — drives the reaction coordinate from separated reactants (0%) through the aromatic-like transition state (~50%) to the fused cyclohexene product (100%); the two forming C–C bond lengths shrink from ≈3.6 Å to a bonded ≈1.54 Å.
- Dienophile — an electron-withdrawing group (EWG, e.g. the nitrile of acrylonitrile) lowers the dienophile's LUMO, shrinking the HOMO(diene)–LUMO(dienophile) gap and lowering Ea — the normal-electron-demand Diels-Alder rate enhancement used throughout synthesis.
- Endo / Exo — the substituent can tuck under the diene π system (endo) or point away (exo). Endo is favoured kinetically by secondary orbital interactions (lower Ea, Alder's endo rule) even though the exo product is often thermodynamically more stable — a classic kinetic-vs-thermodynamic control example.
- Temperature — feeds the Arrhenius equation directly; the rate constant readout updates live as Ea (from substituent + endo/exo) and T change.
Real-world relevance: Diels-Alder cycloadditions build six-membered rings stereospecifically in one step and are a workhorse of total synthesis — from steroid and terpenoid frameworks to Diels-Alder "click"-style bioconjugation.