Halide ion trapping of nitrenium ions formed in the Bamberger rearrangement of N-arylhydroxylamines. Lifetime of the parent phenylnitrenium ion in water

Article abstract of DOI:10.1139/v96-147

The data of p-aminophenol, the product of Bamberger rearrangement, Were analyzed by a mechanism involving rate-limiting formation of the appropriate arylnitrenium ion followed by product-determining steps in involving trapping by the solvent or by the added halide. The possibility that a portion of the halide-trapped products were derived from a pre-association mechanism was also include. Kinetic analyses then produced k_Br:k_w and k_Cl:k_w ratios for two limiting cases, one involving pre-association with an equilibrium constant K_as = 0.3, and one ignoring pre-association. From an azide:water ratio (k_Az:k_w) previously determined for the 2,6-dimethylphenylnitrenium, k_Br was concluded to lie in the range (4-5) × 10⁹ M^-1 s^-1 for all of the nitrenium ions of this study. This range for k_Br then led to k_w values of (1-2) × 10⁹ s^-1 (2,5-Me₂), (2-3) × 10⁹ s^-1 (2-Me), and (4-8) × 10⁹ s^-1 (parent and 2-Cl), where the ranges reflect uncertainties in the exact value of k_Br and in the contribution from pre-association. The lifetime of the parent phenylnitrenium ion in water at one molar ionic strength is concluded to lie in the range 125-250 ps.