Abstract
During the photochemical nitration of phenol, 3,5-dimethylphenol and 1,2-dimethoxybenzene (8) with15N-enriched tetranitromethane, the15N nuclear magnetic resonance (NMR) signals of the nitro products 2-nitrophenol, 4-nitrophenol, 2-nitro-3,5-dimethylphenol, 4-nitro-3,5-dimethylphenol and 1,2-dimethoxy-4-nitrobenzene (9) appear in emission. The nuclear polarizations are built up in radical pairs formed by radical cations or in case of the phenols by phenoxyl radicals and⋅NO2. They are generated by photoreactions from excited triplet states of the reactants or by free radical encounters. With8, 1,2-dimethoxy-4-trinitromethylbenzene is also formed which slowly converts to9. In contrast to this, the15N NMR signals of the nitration products of anisole (11) and 3,5-dimethylanisole (12), 2-nitroanisole, 4-nitroanisole, 3,5-dimethyl-2-nitroanisole and 3,5-dimethyl-4-nitroanisole appear in enhanced absorption indicating the appearance of singlet radical pairs [11 +⋅,⋅NO2]S and [12 +⋅,⋅NO2]S in which the nuclear polarizations are built up. The singlet radical pairs are formed by decomposition of an unstable intermediate, most probably a nitro-trinitromethyl adduct.
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Schürmann, K., Lehnig, M. Mechanistic studies of the photochemical nitration of phenols, 1,2-dimethoxybenzene and anisoles with tetranitromethane by15N CIDNP. Appl. Magn. Reson. 18, 375–384 (2000). https://doi.org/10.1007/BF03162151
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DOI: https://doi.org/10.1007/BF03162151