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Diels-Alder Reaction: [4+2] Cycloaddition

By Chemca Editorial Team • Last Updated: January 2026 • 12 min read

The Diels-Alder Reaction is a concerted pericyclic reaction between a conjugated diene ($4\pi$ electron system) and a substituted alkene, called the dienophile ($2\pi$ electron system), to form a substituted cyclohexene system. It is a [4+2] Cycloaddition reaction.

1. General Reaction

The reaction forms two new sigma ($\sigma$) bonds and one new pi ($\pi$) bond in a single step.

$$ \underbrace{\text{Diene}}_{\text{1,3-Butadiene}} + \underbrace{\text{Dienophile}}_{\text{Ethene}} \xrightarrow{\Delta} \underbrace{\text{Cyclohexene}}_{\text{Adduct}} $$

Key Requirements:

• Diene: Must be able to adopt the s-cis conformation.
• Dienophile: Usually contains Electron Withdrawing Groups (EWG) (e.g., $-CHO, -CN, -COOH$) to increase reactivity.
• Mechanism: Concerted (one-step), cyclic transition state. No intermediates.

2. Reactivity Factors

Electronic Effects

The reaction is fastest when the Diene is Electron-Rich (has EDG like $-Me, -OMe$) and the Dienophile is Electron-Poor (has EWG like $-NO_2, -C=O$). This lowers the HOMO-LUMO energy gap.

Conformation of Diene

Only the s-cis conformation can react. Dienes locked in s-trans conformation (e.g., trans,trans-2,4-hexadiene in a rigid ring system) cannot undergo the reaction.

3. Stereochemistry

The Diels-Alder reaction is highly stereospecific.

A. Syn Addition

The stereochemistry of the dienophile is retained.

• Cis-dienophile $\rightarrow$ Cis-substituted cyclohexene.
• Trans-dienophile $\rightarrow$ Trans-substituted cyclohexene.

B. The Endo Rule

When forming bicyclic systems (e.g., reaction with cyclopentadiene), the Endo product is the major Kinetic Product. The substituent on the dienophile points towards the $\pi$-system of the diene (due to secondary orbital interactions).

Maleic Anhydride + Cyclopentadiene $\rightarrow$ Endo-Adduct (Major)

4. Regiochemistry (Orientation)

If both the diene and dienophile are unsymmetrical, two constitutional isomers are possible.

• 1-Substituted Diene + Dienophile: Generally gives the "Ortho" (1,2) product as major.
• 2-Substituted Diene + Dienophile: Generally gives the "Para" (1,4) product as major.

(Note: "Ortho" and "Para" refer to relative positions 1,2 and 1,4, not benzene derivatives.)

5. Examples

1. Benzyne as Dienophile

Furan + Benzyne $\rightarrow$ Naphthalene endoxide (Diels-Alder adduct).

2. Retro Diels-Alder

Upon heating to high temperatures, cyclohexene derivatives can break back down into diene and dienophile (e.g., cracking of Dicyclopentadiene).

Diels-Alder Quiz

Test your concepts on Cycloadditions. 10 MCQs with explanations.

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