Thermal and Photochemical Nitration of Aromatic Hydrocarbons with Nitrogen Dioxide

Abstract

Aromatic hydrocarbons (ArH) are readily nitrated by nitrogen dioxide (NO 2 ) in dichloromethane at room temperature and below (in the dark). The red colors, transiently observed, arise from the metastable precur- sor complex [ArH, NO + ]NO 3 - , which is formed in the prior dispropor- tionation of nitrogen dioxide induced by the aromatic donor (eq 7). The deliberate irradiation of the diagnostic (red) charge-transfer absorp- tion band (hv CT ) of [ArH, NO + ]NO 3 - at low temperatures results di- rectly in aromatic nitration, even at -78 o C, where the thermal nitra- tion is too slow to compete. The mechanism of the photochemical (charge- transfer) nitration is established by time-resolved laser spectrosco- py to proceed via the aromatic cation radical (ArH .+ ) formed sponta- neously upon the charge-transfer excitation of [ArH,NO + ]NO 3 - in scheme 1. The related thermal activation of [ArH,NO + ]NO 3 - 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 conventially carried out with nitric acid (in the pre- sence of nitrous acid) is delineated in Scheme 4