PHOTOCHEMICAL NITRATION BY TETRANITROMETHANE .10. A POSSIBLE ADDITION-ELIMINATION PATHWAY TO TRINITROMETHYLAROMATIC COMPOUNDS

The photolysis of tetranitromethane and 4-chloroanisole in dichloromethane at +20, -20 and -50-degrees-C led to the formation of mainly 4-chloro-2-trinitromethylanisole (40-50% yield) and a pair of adducts (approximately 30% yield) in which nitrito/trinitromethyl addition had taken place across the 2,5-positions of 4-chloroanisole. Small amounts of a pair of labile adducts (23% yield), assigned the structure of the analogous hydroxy/trinitromethyl adducts were formed in the beginning of the runs. In acetonitrile, the major product from the same reaction was 4-chloro-2-nitroanisole at all three temperatures. Only at -50-degrees-C were the nitrito/trinitromethyl adducts detectable in low yield (12%). The pair of nitrito/trinitromethyl adducts spontaneously decomposed in acetonitrile to give 4-chloro-2-trinitromethylanisole with rate constants of 7 x 10(-4) min-1 at 0-degrees-C and 0.020 min-1 at 20-degrees-C, implying that they should be stable in this solvent at -20 and -50-degrees-C, if formed. The adducts also decomposed to 4-chloro-2-trinitromethylanisole under photochemical conditions, accounting for the formation of this compound in low yield also in acetonitrile. The nitration process predominantly observed in acetonitrile presumably occurs via coupling of 4-chloroanisole radical cation and NO2, both formed in the initial photochemical step. The nitrito/trinitromethyl adducts were thermally and photochemically stable in dichloromethane under the reaction periods employed for photolysis. The conversion into labile hydroxy/trinitromethyl adducts was assumed to occur via acid-catalysed hydrolysis, eventually leading to 4-chloro-2-trinitromethylanisole via acid-promoted elimination of water from the latter adducts. However, only a minor part of this compound can be accounted for in this way.