Maleic anhydride (a dienophile whose two carbonyl groups are cis to each other) reacts with a conjugated diene in a Diels-Alder reaction. What stereochemical relationship will the two carbonyl groups have in the cyclohexene product?
AThey become trans — ring formation through two new bonds inverts one face of the dienophile
BThey remain cis — the concerted suprafacial mechanism freezes the dienophile's geometry into the product
CA mixture of cis and trans is formed because the ring can adopt two chair conformations
DThe reaction does not proceed stereospecifically; the carbonyl relationship is unpredictable
The Diels-Alder reaction is concerted and suprafacial on both components — both new σ bonds form simultaneously on the same face of the dienophile. This means all stereochemical relationships present in the dienophile are perfectly preserved in the product. Cis substituents stay cis; trans substituents stay trans. This predictable stereospecificity is one of the reaction's greatest synthetic strengths: you can design the product's stereochemistry by choosing the dienophile geometry.
Question 2 Multiple Choice
Which diene would you expect to be most reactive in a Diels-Alder reaction with a standard electron-poor dienophile?
A(E,E)-2,4-hexadiene in its preferred s-trans conformation — the most stable conformation reacts fastest
BA flexible acyclic diene with large substituents that strongly prefer the s-trans conformation
CCyclopentadiene, which is locked permanently in the s-cis conformation
DAn isolated (non-conjugated) diene, since isolated double bonds avoid conformational restrictions
The Diels-Alder reaction requires the diene in the s-cis conformation — both double bonds curling toward the same side to overlap simultaneously with both ends of the dienophile. Cyclopentadiene is permanently locked in the s-cis geometry by the ring, which is why it is exceptionally reactive, famously dimerizing on its own at room temperature. Dienes that strongly prefer s-trans (option B) are poor Diels-Alder partners — not just slow but essentially unreactive, because the reactive conformation is almost never populated.
Question 3 True / False
The endo rule predicts the kinetically preferred Diels-Alder product, not necessarily the thermodynamically most stable product.
TTrue
FFalse
Answer: True
The endo product is stabilized by secondary orbital interactions in the transition state — favorable overlap between the dienophile's electron-withdrawing groups and the diene's π system. This lowers the endo transition state energy relative to the exo, making the endo product the kinetic product. However, the exo product is often less sterically crowded and more thermodynamically stable. At higher temperatures or under reversible conditions (retro-Diels-Alder equilibration), the thermodynamic (exo) product can dominate.
Question 4 True / False
A diene locked in the s-trans conformation can still undergo Diels-Alder reactions, but reacts more slowly than a diene in the s-cis conformation.
TTrue
FFalse
Answer: False
This is a critical misconception. The s-trans diene does not merely react slowly — it cannot react at all. The s-cis conformation is required for both termini of the diene to simultaneously overlap with both ends of the dienophile. In the s-trans conformation, the terminal carbons of the diene are too far apart and point in the wrong direction to form both new bonds. It is a geometric impossibility, not a kinetic penalty.
Question 5 Short Answer
Why does the concerted, suprafacial mechanism of the Diels-Alder reaction make it exceptionally useful for stereocontrolled synthesis?
Think about your answer, then reveal below.
Model answer: Because both new σ bonds form simultaneously on the same face of each reactant, all stereochemical relationships present in the starting materials are preserved exactly in the product. A cis dienophile gives a product with cis substituents; a trans dienophile gives trans. This predictability means chemists can design the stereochemistry of the product by selecting the appropriate diene and dienophile geometry — the reaction does not scramble, invert, or racemize the stereocenters it creates.
This stereospecificity, combined with the endo rule's additional selectivity, means the Diels-Alder can create up to four stereocenters in a single step with predictable relative configuration. That is why it is described as one of the most powerful reactions in synthesis: it builds molecular complexity (a six-membered ring with defined stereocenters) from simple starting materials in one pot, without racemization or equilibration.