The highly reactive triple bond of benzyne is a powerful dienophile. Predict the product of the Diels–Alder reaction of benzyne (from chlorobenzene and NaOH, heated) with cyclopentadiene.

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Identify the reactants: Benzyne is generated in situ from chlorobenzene and NaOH under heat. Cyclopentadiene is the diene in this Diels–Alder reaction.

Understand the Diels–Alder reaction mechanism: This is a [4+2] cycloaddition reaction where the diene (cyclopentadiene) reacts with the dienophile (benzyne) to form a six-membered ring.

Analyze the structure of benzyne: Benzyne has a triple bond within a benzene ring, making it highly reactive as a dienophile. The triple bond will participate in the reaction.

Draw the reaction: Orient cyclopentadiene so that its conjugated double bonds align with the triple bond of benzyne. The π-electrons from the diene and dienophile will interact to form two new σ-bonds, creating a bicyclic product.

Predict the product: The product will be a bicyclic compound with a six-membered ring fused to the original cyclopentadiene structure. Ensure to account for stereochemistry if applicable.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Diels–Alder Reaction

The Diels–Alder reaction is a [4+2] cycloaddition reaction between a conjugated diene and a dienophile, resulting in the formation of a six-membered ring. This reaction is significant in organic synthesis due to its ability to create complex cyclic structures in a single step. The reactivity of the diene and dienophile, as well as the stereochemistry of the products, are crucial factors in predicting the outcome of the reaction.

Benzyne

Benzyne is a highly reactive intermediate that contains a triple bond between two carbon atoms in a benzene ring. It can be generated from chlorobenzene through dehydrohalogenation, typically using strong bases like NaOH. Due to its unique structure, benzyne acts as a potent dienophile in Diels–Alder reactions, allowing it to react with various dienes, such as cyclopentadiene.

Cyclopentadiene

Cyclopentadiene is a five-membered cyclic diene that is commonly used in Diels–Alder reactions due to its high reactivity. It has two double bonds that can participate in the cycloaddition process, making it an effective partner for dienophiles like benzyne. The structure and electron-rich nature of cyclopentadiene enhance its ability to form stable adducts with reactive dienophiles, leading to diverse synthetic applications.

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