The rearrangement of aromatic nitro compounds. Part 3. The mechanism of rearrangement of nitrated hydrocarbons in trifluoromethanesulphonic acid

1,3-Dialkyl-2-nitrobenzenes (C₆H₃R₂NO ₂; R = Me, Et, and Prⁱ) rearrange in trifluoromethanesulphonic acid to the corresponding 4-nitro derivatives; with R = Et, this reaction is accompanied by dehydration to 7-ethyl-3-methylanthranil. The reaction rate increases markedly with the size of the alkyl group; with R = Me, the reaction was studied at 110 °C but, with R = Prⁱ, temperatures of 36-54 °C were used. The studies with R = Me show the reaction to be first-order with the rate coefficients (k₁) increasing rapidly with the acidity of the solution [d(log k₁)/d(-H ₀)] = 1.45, decreasing with acidity to 0.49. Double-labelling experiments with ²H and ¹⁵N show the reaction to be intramolecular. 1,2,4-Trimethyl-3-nitrobenzene also rearranges under these conditions to give mainly the 5-nitro isomer. The above results are discussed in terms of a direct 1,3-shift of the nitro group.