Question

3. Predict the relative rates for the reaction of butadiene with ethylene, methyl propenoate and maleic anhydride

Answer 1

Order of reactivity (fastest → slowest):
maleic anhydride >> methyl propenoate (methyl acrylate) > ethylene.

Why:
- Diels–Alder rates are controlled by the HOMO(diene)–LUMO(dienophile) gap. Electron‑withdrawing substituents on the dienophile lower its LUMO and so increase the orbital overlap and rate.
- Maleic anhydride is a very strong electron‑withdrawing, cyclic dienophile (and is locked in the cis geometry required for the reaction), so it reacts fastest and often at or near room temperature.
- Methyl propenoate (methyl acrylate) is moderately activating (ester group), so it reacts faster than unsubstituted ethylene but much more slowly than maleic anhydride.
- Ethylene has no activating group (highest LUMO) and is the least reactive, requiring high temperature/pressure to cycloadd to butadiene.

(Also note maleic anhydride typically shows a strong endo preference due to secondary orbital interactions.)