Kinetics and mechanisms of the Bamberger rearrangement. Part 3. Rearrangement of phenylhydroxylamines to p-aminophenols in aqueous sulphuric acid solutions

Abstract

The rates of Bamberger-type rearrangement of phenylhydroxylamine (1a) and its derivatives (1b–d) to the corresponding p-aminophenols were determined in sulphuric acid solution. Plots of the pseudo-first-order rate constants against pH (or H₀) indicate that the active species at [H₂SO₄] < 1.00N(pH region) is the O-protonated arylhydroxylamine (ArNHO⁺H₂) which exists in equilibrium with the N-protonated species (ArN⁺H₂OH), while the diprotonated species (ArN⁺H₂O⁺H₂) contributes significantly to the observed reaction rate at [H₂SO₄] > 1.00N(H₀ region). The slope of Hammett plots (ρ) which was obtained by plotting k_obs at [H₂SO₄] 1.00N against the Hammett σ values was –3.19. It was also established that the ΔS^‡ values for the four substrates are all positive. These results suggest that the Bamberger rearrangement occurs by an S_N1 mechanism and that the elimination of water from ArNHO⁺H₂ is rate determining.