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 a stepwise mechanism, which would show two separate energy peaks instead of this diagram's single smooth hump.
Diene (HOMO) + Dienophile (LUMO) → [4+2] TS → Cyclohexene
Gap = 9.0 − 3.0·(EDG/100) − 3.0·(EWG/100) eV
Ea = 4 + 3.5·(Gap − 3.0) kcal/mol
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, and both bonds are drawn forming at the same rate — concerted, not stepwise.
- Diene donor strength (EDG) — an electron-donating group on the diene (e.g. an alkoxy substituent) raises the diene's HOMO. This is normal-electron-demand FMO theory: raising HOMO(diene) narrows the HOMO(diene)–LUMO(dienophile) gap.
- Dienophile acceptor strength (EWG) — an electron-withdrawing group on the dienophile (e.g. a nitrile or carbonyl) lowers its LUMO, likewise narrowing the same frontier-orbital gap.
- Both sliders feed one shared, computed HOMO–LUMO gap. A narrower gap gives stronger frontier-orbital overlap at the transition state, which is modeled here as a lower activation energy Ea and, through the Arrhenius equation, a faster rate — the rate-vs-strength chart shows this rising monotonically as either substituent is pushed further.
- Temperature — feeds the Arrhenius equation directly; the rate constant readout updates live as Ea (from the gap) 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. Electron-rich dienes paired with electron-poor dienophiles ("normal electron demand") are the classic textbook rate-acceleration recipe reproduced here.