Electrophilic aromatic substitution. Part 32. The nitration of 1,3,5-trichloro-2-nitrobenzene, 1,3,5-trichloro-2,6-dinitrobenzene, and 1,2,3,5-tetrachloro-4,6-dinitrobenzene in sulphuric acid and in oleum

Abstract

The kinetics of nitration of 1,3,5-trichloro-2-nitrobenzene (TCNB) in sulphuric acid and in oleum at 25, 50, and 75 °C, and of 1,3,5-trichloro-2,4-dinitrobenzene (TCDNB) in oleum at 150 °C have been measured. 1,2,3,5-Tetrachloro-4,6-dinitrobenzene (TetCDNB) reacts with nitric acid in sulphuric acid and in oleum at 150 °C; the rate profile for the reaction shows a nitration process to be occurring. From TCNB in sulphuric acid the yield of TCDNB was not quite quantitative and at 25 °C in oleum a small amount of 1,2,3,5-tetrachloro-4-nitrobenzene was also formed. TCDNB gave a mixture of TCTNB (the major product) and TetCDNB in proportions which varied with acidity; lower acidities were more favourable to the formation of TetCDNB. The total amount of chlorine present in the TCDNB, TCTNB, and TetCDNB was equal to that present in the TCDNB at the start of the reaction. The nitration of TetCDNB in sulphuric acid at 150 °C gave carbon dioxide and hydrogen chloride, and some sulphur dioxide and nitric oxide or nitrogen dioxide. In the nitration of TCDNB this compound produces the chlorinating agent and reacts with it to give TetCDNB. The chlorinating agent can chlorinate added 1,3-dinitrobenzene. The use of H¹⁵NO₃ in the nitration of TCDNB leads to ¹⁵N-incorporation in it and in the TetCDNB produced. Separate nitration of TetCDNB causes only slight ¹⁵N-incorporation into it. It is suggested that in addition to the main route of nitration leading to TCTNB, the nitration of TCDNB gives an ipso-Wheland intermediate by attack of nitronium ion at a nitrated position; this intermediate may either return to starting material or give a diene by nucleophilic capture. Decomposition of the diene gives hydrogen chloride or chloride anion which is oxidised by nitric acid to an electrophilic chlorinating agent.