PHOTOCHEMICAL NITRATION BY TETRANITROMETHANE .19. THE COMPETITIVE REACTIONS OF TRINITROMETHANIDE AND NITROGEN-DIOXIDE WITH RADICAL CATIONS AND THEIR USE FOR SELECTIVE NITRATIONS

The photolysis of the charge-transfer complex between an aromatic compound (ArH) and tetranitromethane is known to form initially a triad of the aromatic radical cation, trinitromethanide ion and NO2 [eqn. (i)]. For reactive and moderately reactive radical cations, the chemical follow-up [GRAPHICS] reactions from the species of the triad are fast at -60-degrees-C, as shown by the fact that the solutions are EPR-silent during photolysis. However, by conducting the photolysis in the presence of a protic acid, the trinitromethanide ion is rendered unreactive by protonation, resulting in the build-up of a detectable (EPR) concentration of ArH.+ or (ArH)2.+. This shows that the initial chemical step from the triad is the nucleophilic attack of trinitromethanide ion upon ArH.+, and that the rate of the reaction between the latter and NO2 must be significantly lower. Preparative experiments support this conclusion, in that the predominant adduct formation from ArH-tetranitromethane photolysis is diverted into nitro substitution in the presence of a protic acid, the latter reaction occurring via ArH.+-NO2 coupling. These findings also establish that results obtained from the photonitration of aromatics by tetranitromethane are not relevant for judging the possible electron transfer nature of electrophilic aromatic nitration by nitronium ion.