Kinetics and mechanism of the Fischer–Hepp rearrangement and denitrosation. Part VII. Reactions at high acidity

Abstract

Rate constants for the denitrosation and also for the rearrangement of N-methyl-N-nitrosoaniline have been obtained in acid solution up to 10M-HCl and 12M-H₂SO₄. For denitrosation the shape of the rate–acidity profile changes as the acidity of the medium is increased; the zero-order behaviour in X (the added nitrite trap which ensures the irreversibility of denitrosation) is lost and the shape of the plot becomes dependant upon the nature and concentration of X. For X = HN₃ and NH₂NH₂ a change to a smaller slope occurs at ca. 4.5M-HCl, whereas for X = NH₂OH, CO(NH₂)₂ and NH₂SO₄H there is a maximum value of the observed rate constant k₀, and a decreasing value at higher acidities. The results are interpreted in terms of the possible protonation equilibria involving some of the X species at high acidities, with a consequent loss of reactivity towards electrophilic nitrosation. The rate constant for rearrangement levels off at ca. 9.7M-H₂SO₄, and thereafter decreases with increasing acidity. This is discussed in terms of the (partly rate-determining) final proton transfer from the para-position in the nitroso-amine to the solvent.