OZONE-MEDIATED NITRATION OF CHLORO-BENZENE AND BROMO-BENZENE AND SOME METHYL-DERIVATIVES WITH NITROGEN-DIOXIDE - HIGH ORTHO-DIRECTING TRENDS OF THE CHLORINE AND BROMINE SUBSTITUENTS

In the presence of ozone, chloro- and bromo-benzenes are nitrated smoothly with nitrogen dioxide at low temperatures, giving a mixture of the corresponding nitro derivatives in nearly quantitative yield. The nitration products are generally ortho-rich as compared with those obtained by the conventional procedures based on the use of nitric acid or mixed acid. Interestingly. the ortho:para isomer ratios of the products can be reversed from ortho-rich (o:p = 1.14 and 1.09) to parapredominant to (o:p = 0.45 and 0.68) simply by altering the initial concentration of the substrate. This enigmatic phenomenon has been interpreted in terms of the equilibrium between the monomer and the dimer forms of the cation radical derived from a halogenobenzene and the difference in their relative reactivity toward nitrogen dioxide. Similar preference for substitution ortho to the halogen substituent has been observed with 4-chloro- and 4-bromo-toluenes, which concurrently suffer extensive ipso attack by the present reagent system, leading to the formation of 4-methyl-2-nitrophenol as a common side product.