Nitrosation by nitrosyl halides

Solutions of nitrous acid in aqueous hydrochloric acid or hydrobromic acid give rise to an equilibrium concentration of the corresponding nitrosyl halides, as indicated in (14). At 25°C, the equilibrium constants for the formation of 

6 A recent review of the reactivity of aliphatic amines towards nitrous anhydride (Mirvish, 1975), provides evidence that these reaction rates are curiously insensitive to the basicity of the substrate even though the overall reaction rate is much less than the encounter rate. The reason for this is not clear. 

The first evidence for the involvement of these nitrosyl halides in nitrosation came from kinetic studies of diazotization. The rate equation for diazotization in aqueous hydrochloric and hydrobromic acids includes the kinetic term given in (15) where S stands for the free amine (Schmid, 1937 Schmid and Muhr, 1937). It was later recognized that this can be interpreted as the attack of the nitrosyl halides on the free amine (Hammett, 1940) and the kinetic term was rewritten as (16).7 The corresponding rate coefficients and activation energies calculated from the equilibrium concentration of the nitrosyl halides are given in Tables 3 and 4 together with later results (Williams, 1977). 

The extent to which the rate coefficients in Tables 3 and 4 approach that expected for a diffusion-controlled process is illustrated in Fig. 1. For the less reactive amines, nitrosyl chloride reacts significantly faster than nitrosyl 

7 Because of the number of rate equations in this chapter, the symbol k without subscripts or superscripts is used to designate any rate coefficient. A particular rate coefficient is then specified by reference to the equation by which it is defined. 

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Most of the mechanistic work concerning nitrosation by nitrosyl halides, has involved the in situ formation of low equilibrium concentrations of the nitrosyl halides by addition of halide ion to acid solutions of nitrous acid (17). The nitrosyl halides can be detected spectrophotometrically and the equilibrium constants t>een determined at several different temperatures for... 

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