12. Which of the following dienes does not react with the dienophile in the Diels-Alder
reaction?
a. A
b. B
c. C
d. D

The diene that does not react in the Diels–Alder reaction is option D (the isolated diene in cyclopentane).

Key concept: Diels–Alder reactive dienes

For a Diels–Alder reaction, the diene must be conjugated (1,3‑diene) and able to adopt the s‑cis conformation; dienes that are “locked” so they cannot become s‑cis, or that are not conjugated, are essentially unreactive. Conjugation allows the four π‑electrons of the diene to participate as a unit, and the s‑cis conformation brings the terminal carbons close enough to the dienophile to form the new σ‑bonds.​

Option A: Reactive cyclic conjugated diene

Structure A is a six‑membered ring containing a 1,3‑diene with a methyl substituent on the ring. The double bonds are conjugated and the ring locks them in the necessary s‑cis geometry, making A an excellent Diels–Alder diene. Substituents such as a methyl group on the ring do not stop the reaction; they only influence rate and stereochemistry.​

Option B: Reactive acyclic conjugated diene

Structure B is an open‑chain 1,3‑diene (like a substituted butadiene) with conjugated double bonds. Acyclic conjugated dienes freely rotate and can adopt the reactive s‑cis conformation, so they readily undergo Diels–Alder cycloaddition with suitable dienophiles. Any alkyl substitution on the chain here is not bulky enough to prevent the required rotation, so B is reactive.​

Option C: Reactive cyclic s‑cis diene

Structure C is a cyclohexadiene derivative in which the two double bonds are conjugated and locked in an s‑cis arrangement. Cyclic conjugated cis‑dienes of this type are among the most reactive dienes in Diels–Alder chemistry, because the ring pre‑organizes the π‑system and reduces the entropic cost of cycloaddition. The exocyclic double bond in C is part of the conjugated 1,3‑diene and participates fully in the reaction.​

Option D: Non‑reactive non‑conjugated diene

Structure D is a five‑membered ring with a single double bond and another double bond outside the ring that is separated by more than one σ‑bond; these π‑bonds are not conjugated and cannot act together as a 1,3‑diene. Because the π‑electrons are not part of a conjugated 4‑π‑electron system and cannot adopt a continuous s‑cis 1,3‑diene geometry, D does not undergo the normal Diels–Alder reaction, making D the correct answer.​

Short SEO‑oriented introduction

Understanding which of the following dienes does not react with the dienophile in the Diels–Alder reaction is a common test of conceptual clarity in organic chemistry exams. By focusing on conjugation and the ability of a diene to adopt the crucial s‑cis conformation, this question highlights the fundamental structural features that control Diels–Alder reactivity.