Q2(b). Benzyne from anthranilic acid; trapping with furan and benzoic acid

Reading of the question: “Amyl nitrate” is read as amyl nitrite (isopentyl nitrite), the reagent that diazotises anthranilic acid in organic solvents.

Introduction. Anthranilic acid (2-aminobenzoic acid) is the standard laboratory source of benzyne. Diazotisation gives an inner salt that loses two stable small molecules on warming. Benzyne is trapped as it forms.

Formation of X (benzyne)

Scheme: The zwitterion fragments in one step; two new stable molecules (N₂, CO₂) drive the reaction.

1. Amyl nitrite converts -NH₂ into -N₂⁺ without water; the carboxylic acid becomes carboxylate, so the product is the neutral inner salt benzenediazonium-2-carboxylate.
2. On warming it loses N₂ and CO₂ from adjacent carbons: X = benzyne (1,2-didehydrobenzene, C₆H₄).
3. The extra bond of benzyne is formed by side-on overlap of two sp² orbitals in the plane of the ring, outside the aromatic π system. The overlap is poor, so the bond is weak and benzyne is a very reactive electrophile and dienophile, with a lifetime too short for isolation.

Y: Diels–Alder with furan

Scheme: Furan acts as the diene, benzyne as the dienophile; the oxygen becomes a one-atom bridge.

1. Furan is an s-cis diene; benzyne adds across C-2 and C-5 in a concerted, thermally allowed cycloaddition.
2. Y = 1,4-dihydro-1,4-epoxynaphthalene (1,4-epoxy-1,4-dihydronaphthalene, C₁₀H₈O). Isolation of this adduct is the classic trapping evidence for benzyne.

Z: addition of benzoic acid

Scheme: Benzoate adds to one carbon of the strained bond; the aryl anion takes a proton.

1. The carboxylate oxygen of benzoic acid adds as a nucleophile to one carbon of benzyne (nucleophilic addition), giving an aryl carbanion that is protonated by C₆H₅COOH.
2. Z = phenyl benzoate, C₆H₅COOC₆H₅ (C₁₃H₁₀O₂).

Conclusion. X = benzyne; Y = 1,4-dihydro-1,4-epoxynaphthalene (Diels–Alder with furan); Z = phenyl benzoate (nucleophilic addition of benzoate).

Application. Benzyne cycloadditions build polycyclic aromatics (triptycene from anthracene); the furan adduct Y is deoxygenated to naphthalene derivatives and used to make 1-naphthol.