Kinetics and mechanism of the Fischer–Hepp rearrangement and denitrosation. Part 10. Reactions of 3-methoxy-N-methyl-N-nitrosoaniline

Abstract

Further evidence is presented in favour of an intramolecular mechanism for the acid-catalysed Fischer–Hepp rearrangement of aromatic N-nitrosamines, both in water and in ethanol solvent. In aqueous acid 3-methoxy-N-methyl-N-nitrosoaniline (IV) gives the rearrangement product 3-methoxy-N-methyl-4-nitrosoaniline (V) in >90% yield. This figure drops a little to a constant 85% in the presence of one of three conventional ‘nitrite traps’, hydrazoic acid, hydrazine, and sulphamic acid, and over a five-fold range of concentration of each. Additionally the observed first-order rate constant (k0) is the same in the whole series. When nucleophilic species [Cl–, Br–, SCN–, and SC(NH2)2] are added the yield of (V) drops and the product of denitrosation, 3-methoxy-N-methylaniline (VI), is also observed. The effect is most marked for the most powerful nucleophiles[SCN– and SC(NH2)2]. At the same time kO increases with increasing nucleophile concentration. The results are wholly consistent with a mechanism involving parallel reactions of the protonated N-nitrosamine to give either the product of denitrosation (by nucleophile attack) which is normally reversible, or that of rearrangement where the nucleophile is not involved, and which is irreversible. The results cannot be accounted for in terms of the older intermolecular mechanism whereby the rearrangement product arises by conventional electrophilic C-nitrosation involving both the products of denitrosation. This implies strongly that the rearrangement is an intramolecular process. Similar results are reported for the variation of yields and rate constants with added nucleophiles for the same reaction in acidified ethanol, so that it is very likely that the same mechanism operates in that solvent.