Electrophilic aromatic substitution. Part 36. Kinetics of aromatic nitrations in solutions of dinitrogen pentaoxide and of nitronium salts in nitric acid

Abstract

Kinetics of nitration of some aromatic substrates in nitric acid solutions containing dinitrogen pentaoxide or nitronium salt have been compared. Nitronium trifluoromethanesulphonate was the salt preferred for reasons of stability and solubility. In most cases rate coefficients for nitration in dinitrogen pentaoxide solutions were similar to those in nitronium salt solutions of the same estimated nitronium ion concentration; normal nitronium ion nitration is indicated. Previously recognised rate enhancements at high concentrations are largely medium effects. With 1,2,4-trichloro-5-nitrobenzene there is evidence for the intrusion of an extra mechanism for reaction in highly concentrated solutions of dinitrogen pentaoxide. Other substrates studied were 1,2-dichloro-4-nitrobenzene, 1,3-dimethyl-4,6-dinitrobenzene, 2-nitrobenzoic acid, 3-nitrobenzoic acid, 2-chloro-3-nitrobenzoic acid, and 4-chloro-3-nitrobenzoic acid. Relative reactivities among the benzoic acids are discussed.