The Diels-Alder reaction is a [4+2] cycloaddition forming what kind of ring?

• A. A cyclohexene ring
• B. A benzene ring
• C. A cyclopentane ring
• D. A cycloheptatriene ring

Answer: (A)

The main idea is that a Diels-Alder reaction is a [4+2] cycloaddition that combines a diene (four π electrons) with a dienophile (two π electrons) to form a six-membered ring in a single step. The two new sigma bonds close the ring, and the electrons from the diene rearrange to leave one C=C bond in the product. That gives a six-membered ring with a double bond: cyclohexene. The other choices describe rings or features not produced directly by this reaction: a benzene ring would require aromatic stabilization, and cyclohexane, cyclopentane, or cycloheptatriene would correspond to different ring sizes or lack of a double bond shown in the Diels-Alder product.