Electrophilic aromatic substitution. Part 33. Kinetics and products of aromatic nitrations in solutions of dinitrogen pentaoxide in nitric acid
The kinetics and/or products of reaction, in nitric acid containing 0–5 mol dm–3 of dinitrogen pentaoxide, of phenyltrimethylammonium perchlorate, 1,2-dichloro-4-nitrobenzene, 2,4-dinitrotoluene, 1,4-dichloro-2-nitrobenzene, 1,2,4-trichloro-5-nitrobenzene, 1,2,4,5-tetrachlorobenzene, pentachlorobenzene, 1,2,3-trichlorobenzene, 1,3,5-trichloro-2-nitrobenzene, 1,2-dichloro-3-nitrobenzene, 1,2,3,4-tetrachlorobenzene, and 1,3-dichloro-2-nitrobenzene, have been studied. For each substrate investigated kinetically, rate coefficients for nitration in solutions containing more than ca. 2 mol dm–3 dinitrogen pentaoxide increase more quickly than the concentration of nitronium ion. In the cases of 1,2,4,5-tetrachlorobenzene, 1,4-dichloro-2-nitrobenzene, and 1,2,4-trichloro-5-nitrobenzene, but not with the other substrates, there was evidence for the formation of, in addition to expected aromatic nitroproducts, unstable cyclohexadiene products. Those from 1,2,4-trichloro-5-nitrobenzene were identified as a diastereoisomeric pair of 3,5,6-trichloro-2,4-dinitrocyclohexa-2,5-dienyl nitrates. A mechanism for the formation of these products is proposed.