Question

When anthracene is added to the reaction of chlorobenzene with concentrated NaOH at 350 °C, an interesting Diels–Alder adduct of the formula C₂₀H₁₄results. The proton NMR spectrum of the product shows a singlet of area 2 around d 3 and a broad singlet of area 12 around d 7. Propose a structure for the product, and explain why one of the aromatic rings of anthracene reacted as a diene.

Short answer

Formation of benzyne

Formation of adduct

Step-by-step solution

Preparation of benzyne

Benzyne is prepared by the reaction of chlorobenzene with sodium hydroxide at 350 °C. This benzyne acts as a dienophile and anthracene acts as a diene and undergo Diels alder reaction to form the adduct.

Formation of benzyne

Formation of the adduct

In the aromatic ring of anthracene, the whole molecule doesn’t act as diene only the middle ring acts as a diene because an alternate conjugated double bond is present.

Formation of adduct