Free-Radical Nitration of Naphthalene with Nitrogen Dioxide in CCl4 and Implications for Environmental Nitrations

Article abstract of DOI:10.1021/jo00264a009

The nitration of naphthalene (NAP) with nitrogen dioxide in carbon tetrachloride occurs via a free-radical mechanism, involving metastable adducts of NAP and 2-4 mol of NO2 per mole of NAP.This free radical nitration system is characterized by (1) low 1-nitronaphthalene/2-nitronaphthalene (1NNAP/2NNAP) ratios and (2) the formation of unexpected dinitronaphthalene isomers, 1,3-dinitronaphthalene (1,3-diNNAP) and 2,3-dinitronaphthalene (2,3-diNNAP), at low conversions.There is strong steric repulsion of the nitro groups in the ORTEP drawing of the 2,3-diNNAP crystal structure (Figure 1).The elimination of HNO2 from a postulated tetranitrotetrahydronaphthalene intermediate is, therefore, suggested to occur under kinetic control.The nitro substituent has a small activating effect toward free-radical nitration in 2NNAP while it has no noticeable effect in 1NNAP, contrasting sharply with conventional electrophilic nitration where the nitro substituent has a very strong deactivating effect.An ionic electrophilic reaction mechanism predominates in solvents of higher polarity and is subject to efficient acid catalysis.We suggest our conditions of free-radical nitration model the gas-phase atmospheric free-radical nitration of NAP, where low 1NNAP/2NNAP ratios and 1,3-diNNAP also have been reported.Thus, free-radical reactions might be responsible for producing some of the nitro-containing polycyclic aromatic hydrocarbon mutagens that are found in the environment.