Aspects of nitrosation & diazotization

Abstract

In the presence of a relatively high concentration of sodium azide N-nitrosodiphenylamine undergoes irreversible denitrosation in aqueous acidic media to give diphenylamine. The reaction is catalysed by both H(^+) (with k(_o)oc h(_o)) and various nucleophiles, and for which, at low nucleophile concentrations, a solvent-isotope effect of (^k)D/k(_H)= 2.0 is observed. These facts are consistent with rate-determining attack by the nucleophile upon the protonated form of the nitrosamine. At high bromide concentrations k(_o) becomes independent of [Br(^-)] and the solvent-isotope effect changes to(^k)D/k(_H)= 0.8. This suggests that the initial protonation of the nitrosamine then becomes rate-determining. Direct catalysis by aniline and its ring-substituted derivatives is also observed and has been interpreted in terms of π - complex formation between Ph(_2)NH.NO and ArNH(_3). The applicability of such a mechanism to the Fischer-Hepp rearrangement and related reactions is also discussed. In general k(_o) oc 1/[Ph(_2)NH] and a study of this effect has revealed a reactivity sequence for some 'nitrite-traps' toward reaction with free nitrosating species such as NOBr. The diazotization of several substituted anilines has been carried out in aqueous acidic media under conditions such that the active nitrosating agents are NOCL and NOBr. The variation of the observed rate constant (k(_o)) with [Halide] suggests that the process is reversible, particularly at high [Halide] and for those anilines for which pK(_BH)+<4.0, and that this accounts for the earlier observation of an apparently large substituent selectivity for reaction at high [HCI]. This large selectivity was originally taken as evidence against diffusion-control but an analysis of k(_o) in terms of a reversible steady-state scheme has yielded values for the true bimolecular rate constants for encounter between NOX and ArNH(_2) which do indeed approach the diffusion-controlled limit. For those amines carrying electron-withdrawing groups a second relaxation can be observed and it is suggested that this corresponds to the tautomerisation of the intermediate primary nitrosamine carrying electron-withdrawing groups a second relaxation can be observed and it is suggested that this corresponds to the tautomerisation of the intermediate primary nitrosamine.