Kinetics and mechanism of the Fischer–Hepp rearrangement and denitrosation. Part V. The mechanism of denitrosation

Abstract

Rate measurements have been carried out for the reaction of N-methyl-N-nitrosoaniline in hydrochloric acid solution (0·4-5M) containing an excess of sulphamic acid, urea, or sodium azide. Under these conditions denitrosation (to form N-methylaniline) occurs irreversibly and without formation of any significant amounts of the rearrangement product except at low chloride ion concentrations. The first-order rate coefficient is proportional to the product h_o[Cl^–]; this is interpreted as rate-determining nucleophilic attack by chloride ion on the protonated form of the nitrosamine. Similar results are obtained for reaction in sulphuric acid containing sodium chloride and also other nucleophiles. The reaction is very sensitive to the nature of the nucleophile: the following increasing order of reactivity is observed, Cl^–,Br^–,SCN^–,I^–; 1:54 : 5300:15,000. This contrasts with the corresponding reactions of H₂NO₂, which discriminates very little between these nucleophiles. Denitrosation also occurs in sulphuric acid solution containing no added nucleophiles. The rate is now proportional to h₀^1·6, and there is no evidence of any significant attack by the hydrogen sulphate ion. These results are interpreted in terms of two concurrent mechanisms, one involving denitrosation brought about by a water molecule and the other which becomes more important at high acidity, where the reacting entity is H₃O⁺. These mechanisms are discussed with reference to the reverse reaction, i.e. N-nitrosation of aromatic amines.