The mechanism of charge-transfer nitration of naphthalene

Electrophilic (thermal) and charge-transfer (photochemical) nitration of naphthalene are effectively carried out in acetonitrile with various X-substituted N-nitropyridinium salts with X = 4-MeO2, H, 4-MeO2C and 2,6-Me2. Quantitative analyses indicate that both processes effect nuclear nitration to afford the same distribution of isomeric α- and β-nitronaphthalenes, together with the production of various amounts of (nitro-pyridine) adducts to naphthalene. Time-resolved (picosecond) spectroscopy identifies the naphthalene cation radical (NAPH˙+) as the critical reactive intermediate in charge-transfer nitration. The subsequent disappearance of NAPH˙+ occurs by its combination with NO2 to form the isomeric (α/β) Wheland intermediates, which suffer competitive deprotonation (to yield the nitronaphthalenes) and nucleophilic addition (to produce the adducts). The relevance of such a charge-transfer mechanism to naphthalene nitration via the electrophilic (thermal) process is discussed.