THERMAL AND PHOTOCHEMICAL NITRATION OF AROMATIC-HYDROCARBONS WITH NITROGEN-DIOXIDE

Aromatic hydrocarbons (ArH) are readily nitrated by nitrogen dioxide (NO2) in dichloromethane at room temperature and below (in the dark). The red colors, transiently observed, arise from the metastable precursor complex [ArH, NO+]NO3-, which is formed in the prior disproportionation of nitrogen dioxide induced by the aromatic donor (eq 7). The deliberate irradiation of the diagnostic (red) charge-transfer absorption band (h nu(CT)) of [ArH, NO+] NO3- at low temperatures results directly in aromatic nitration, even at -78 degrees C, where the thermal nitration is too slow to compete. The mechanism of the photochemical (charge-transfer) nitration is established by time-resolved laser spectroscopy to proceed via the aromatic cation radical (ArH.+) formed spontaneously upon the charge-transfer excitation of [ArH, NO+]NO3- in Scheme 1. The related thermal activation of [ArH, NO+]NO3- derives from the adiabatic electron transfer that produces the same radical pair [ArH.+ NO.] as the reactive intermediate in Scheme 3. The close relationship between the thermal/photochemical nitrations with nitrogen dioxide and those conventionally carried out with nitric acid (in the presence of nitrous acid) is delineated by Scheme 4.