Kinetics and mechanism of the hydrolysis of N-methyl-N-nitroamides in aqueous sulphuric acid

Abstract

Pseudo first-order rate constants for the hydrolysis of N-methyl-N-nitroacetamide and various 4-substituted N-methyl-N-nitrobenzamides in sulphuric acid solutions are reported. N-Methyl-N-nitroacetamide undergoes an acid-catalysed process at all acidities studied for which the solvent deuterium isotope effect, k₀^H₂SO₄/k₀^D₂SO₄, is 0.87, and ΔS^‡ca.–85 (±10) J K^–1 mol^–1. These results suggest an A_Ac2 mechanism involving rapid pre-equilibrium protonation of the substrate followed by rate-limiting attack of water at the carbonyl C-atom to form a tetrahedral intermediate which collapses, in a fast step, to the products. The N-methyl-N-nitrobenzamides, however, exhibit both non-catalysed and acid-catalysed hydrolysis. The non-catalysed pathway operates at acidities up to ca. 5 mol dm^–3 H₂SO₄, and is characterised by a solvent deuterium isotope effect, k₀^H₂sO₄/k₀^D₂SO₄, of 1.5, ΔS^‡ca.–100 (±10) J K^–1 mol^–1 and a Hammett ρ value of 1.0 (±0.1). No catalysis by Br^– is observed and the results are most consistent with a thermal rearrangement and expulsion of N₂O. The acid-catalysed pathway operates at acidities >5 mol dm^–3 H₂SO₄. The solvent deuterium isotope effect k₀^H₂SO₄/k₀^D₂SO₄ is 0.58, ΔS^‡ > O JK^–1 mol^–1 and the Hammett ρ value is –3.4 (±0.1). Thus, a change in mechanism occurs at ca. 5 mol dm^–3 H₂SO₄ to an Ac1 pathway involving protonation of the substrate, followed by rate-limiting cleavage of the amide C–N bond to form a benzoyl cation. The different acid-catalysed hydrolysis pathways for the N-nitroacetamide and N-nitrobenzamides is ascribed to the stabilisation afforded to the benzoyl cation as opposed to the acetyl carbonium ion. N-Nitroamides are hydrolysed exclusively via amide C–N bond cleavage whereas the corresponding N-nitrosoamides decompose via concurrent C–N and N–N (i.e. denitrosation) bond cleavage. This difference between N-nitro and N-nitroso-amides is discussed in terms of the greater stability of the NO ⁺ group.